[Rietveld] Add support for patchset dependencies

Add support for patchset dependencies.

The following is a summary of my changes:

* A new endpoint "get_depends_on_patchset" has been added. This endpoint will be called by apply_issue to form a chain of dependencies to apply to the local checkout. The apply_issue change is here: https://codereview.chromium.org/1149653002/.

* I extracted majority of the new logic into a new module (dependency_utils) and added comprehensive unit tests.

* This CL adds two new fields to the PatchSet model. These fields define the dependency and the dependents of the patchset.

* The dependency and dependents of a patchset are displayed to the user but only in the new UI. I figure we are close enough to making the new UI the default that I can stop doing double UI work.

* I tried to cover all interactions among dependent patchsets, here is a example of dependencies and how different Rietveld actions will impact those dependencies-

                   -> CL3#PS2
CL1#PS2 -> CL2#PS1 -> CL3.1#PS1
                   -> CL3.1#PS2

Explanation of the above: CL3#PS2 & CL3.1#PS1 & CL3.1#PS2 all depend on CL2#PS1 which depends on CL1#PS2.

==Rietveld action: issue is closed and reopened==
- If CL2 is closed then CL2#PS1 shows up with a strikethrough in CL3#PS2 & CL3.1#PS1 & CL3.1#PS2.
- If CL2 is reopened then CL2#PS1 shows up normally (without a strikethrough) in CL3#PS2 & CL3.1#PS1 & CL3.1#PS2.

==Rietveld action: issue is deleted==
- If CL2 is deleted then CL3#PS2 & CL3.1#PS1 & CL3.1#PS2 are updated to remove their dependency on CL2#PS1.
  Additionally, if any patchset of CL2 is a dependent then its dependency is removed. i.e. CL1#PS2 will be updated to remove CL2#PS1 as a dependent.

==Rietveld action: patchset is deleted==
- If CL2#PS1 is deleted then CL3#PS2 & CL3.1#PS1 & CL3.1#PS2 are updated to remove their dependency on CL2#PS1.
  Additionally, if CL2#PS1 is a dependent then its dependency is removed. i.e. CL1#PS2 will be updated to remove CL2#PS1 as a dependent.


Tested end-to-end using a test Git repository (https://skia.googlesource.com/skiabot-test/) and the following CLs created in my test Rietveld instance:
* https://skia-codereview-staging.appspot.com/931002  ('Branch1 CL')
* https://skia-codereview-staging.appspot.com/5001001 ('Branch2 CL')
* https://skia-codereview-staging.appspot.com/9881001 ('Branch3 CL')
* https://skia-codereview-staging.appspot.com/3951001 ('Branch3.1 CL')
Opt into the new UI and observe the new 'Depends on Patchset' and 'Dependent Patchsets' sections in the above CLs.


Design doc is here: https://docs.google.com/document/d/1KZGFKZpOPvco81sYVRCzwlnjGctup71RAzY0MSb0ntc/edit#heading=h.6r6lt4tsvssw

BUG=502255
Committed: https://chromium.googlesource.com/infra/infra/+/be2d01f56f79233fb2fe216916469ac4c06c24a4

[rmistry, 2015-06-03 12:40:35]

Patchset #18 (id:330001) has been deleted

[rmistry, 2015-06-08 14:05:53]

rmistry@google.com changed reviewers:
+ jrobbins@chromium.org

[jrobbins, 2015-06-09 17:23:08]

lgtm.  Outstanding job on this.  Thanks!

https://codereview.chromium.org/1155513002/diff/430001/appengine/chromium_rie...
File appengine/chromium_rietveld/new_static/components/cr-issue-patchset.html
(right):

https://codereview.chromium.org/1155513002/diff/430001/appengine/chromium_rie...
appengine/chromium_rietveld/new_static/components/cr-issue-patchset.html:74: {{
patchset.dependsOnPatchset | getPatchsetUrl }}
Rather than displaying the patchset URL as the link text, can we make it look
like this?
Issue 12345678 Patch 1

[rmistry, 2015-06-10 12:43:30]

https://codereview.chromium.org/1155513002/diff/430001/appengine/chromium_rie...
File appengine/chromium_rietveld/new_static/components/cr-issue-patchset.html
(right):

https://codereview.chromium.org/1155513002/diff/430001/appengine/chromium_rie...
appengine/chromium_rietveld/new_static/components/cr-issue-patchset.html:74: {{
patchset.dependsOnPatchset | getPatchsetUrl }}
On 2015/06/09 17:23:08, Jason Robbins wrote:
> Rather than displaying the patchset URL as the link text, can we make it look
> like this?
> Issue 12345678 Patch 1

Done. I also staged it on skia-codereview-staging where you can take a look:

* https://skia-codereview-staging.appspot.com/931002  ('Branch1 CL')
* https://skia-codereview-staging.appspot.com/5001001 ('Branch2 CL')
* https://skia-codereview-staging.appspot.com/9881001 ('Branch3 CL')
* https://skia-codereview-staging.appspot.com/3951001 ('Branch3.1 CL')

[jrobbins, 2015-06-10 19:28:30]

lgtm

[rmistry, 2015-06-10 19:58:33]

On 2015/06/10 19:28:30, Jason Robbins wrote:
> lgtm

Jason, this CL only displays dependencies in the new UI. How close are we to
making it the default?
Should I quickly put together something to display in the old UI as well?

[Jason Robbins -- corp, 2015-06-10 23:23:40]

On 2015/06/10 at 19:58:33, rmistry wrote:
> On 2015/06/10 19:28:30, Jason Robbins wrote:
> > lgtm
> 
> Jason, this CL only displays dependencies in the new UI. How close are we to
making it the default?
> Should I quickly put together something to display in the old UI as well?

In terms of calendar time, probably another 4-6 weeks.

I suggest supporting the new UI only, or at least start with that position and
see if users have a good reason to demand old UI support.

[rmistry, 2015-06-11 20:23:25]

On 2015/06/10 23:23:40, Jason Robbins -- corp wrote:
> On 2015/06/10 at 19:58:33, rmistry wrote:
> > On 2015/06/10 19:28:30, Jason Robbins wrote:
> > > lgtm
> > 
> > Jason, this CL only displays dependencies in the new UI. How close are we to
> making it the default?
> > Should I quickly put together something to display in the old UI as well?
> 
> In terms of calendar time, probably another 4-6 weeks.
> 
> I suggest supporting the new UI only, or at least start with that position and
> see if users have a good reason to demand old UI support.

sgtm

[rmistry, 2015-06-19 16:40:24]

On 2015/06/11 20:23:25, rmistry wrote:
> On 2015/06/10 23:23:40, Jason Robbins -- corp wrote:
> > On 2015/06/10 at 19:58:33, rmistry wrote:
> > > On 2015/06/10 19:28:30, Jason Robbins wrote:
> > > > lgtm
> > > 
> > > Jason, this CL only displays dependencies in the new UI. How close are we
to
> > making it the default?
> > > Should I quickly put together something to display in the old UI as well?
> > 
> > In terms of calendar time, probably another 4-6 weeks.
> > 
> > I suggest supporting the new UI only, or at least start with that position
and
> > see if users have a good reason to demand old UI support.
> 
> sgtm

[rmistry, 2015-06-19 16:40:32]

The CQ bit was checked by rmistry@google.com

[commit-bot: I haz the power, 2015-06-19 16:41:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1155513002/450001

[rmistry, 2015-06-19 16:44:53]

The CQ bit was unchecked by rmistry@google.com

[commit-bot: I haz the power, 2015-06-19 16:45:00]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-19 16:45:01]

Try jobs failed on following builders:
  infra_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/infra_presubmit...)

[rmistry, 2015-06-19 16:48:04]

Patchset #24 (id:470001) has been deleted

[rmistry, 2015-06-19 17:03:00]

The CQ bit was checked by rmistry@google.com to run a CQ dry run

[rmistry, 2015-06-19 17:03:00]

The patchset sent to the CQ was uploaded after l-g-t-m from
jrobbins@chromium.org
Link to the patchset: https://codereview.chromium.org/1155513002/#ps500001
(title: "Fix lint issues in test")

[commit-bot: I haz the power, 2015-06-19 17:03:12]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1155513002/500001

[commit-bot: I haz the power, 2015-06-19 17:06:28]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-19 17:06:29]

Dry run: This issue passed the CQ dry run.

[rmistry, 2015-06-19 17:06:46]

The CQ bit was checked by rmistry@google.com

[commit-bot: I haz the power, 2015-06-19 17:07:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1155513002/500001

[commit-bot: I haz the power, 2015-06-19 17:09:10]

Committed patchset #25 (id:500001) as
https://chromium.googlesource.com/infra/infra/+/be2d01f56f79233fb2fe216916469...