Rewrite git gclient sync/update to be simpler and more reliable

Rewrite git gclient sync/update to be simpler and more reliable

This does two main things for the GitWrapper:update() (a.k.a.
gclient sync) routine:
* Linearizes much of the flow, so that early failure cases result
  in aborts rather than wasted effort, and it is easier to understand
* Ensures that a 'fetch' is run early on, so that we can guarantee that
  the target revision is either available, or entirely unfetchable, and
  proceed appropriately.

R=iannucci@chromium.org, maruel@chromium.org
BUG=643346

[agable, 2017-01-19 23:13:19]

The CQ bit was checked by agable@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-01-19 23:13:28]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-19 23:19:56]

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

[commit-bot: I haz the power, 2017-01-19 23:19:57]

Dry run: Try jobs failed on following builders:
  Depot Tools Presubmit on luci.infra.try (JOB_FAILED,
https://luci-milo.appspot.com/swarming/task/33d1d65355ce0710)

[agable, 2017-04-18 18:39:52]

Description was changed from

==========
Rewrite git gclient sync/update to be simpler and more reliable

This does two main things for the GitWrapper:update() (a.k.a.
gclient sync) routine:
* Linearizes much of the flow, so that early failure cases result
  in aborts rather than wasted effort, and it is easier to understand
* Ensures that a 'fetch' is run early on, so that we can guarantee that
  the target revision is either available, or entirely unfetchable, and
  proceed appropriately.

R=iannucci@chromium.org, maruel@chromium.org
BUG=643346
==========

to

==========
Rewrite git gclient sync/update to be simpler and more reliable

This does two main things for the GitWrapper:update() (a.k.a.
gclient sync) routine:
* Linearizes much of the flow, so that early failure cases result
  in aborts rather than wasted effort, and it is easier to understand
* Ensures that a 'fetch' is run early on, so that we can guarantee that
  the target revision is either available, or entirely unfetchable, and
  proceed appropriately.

R=iannucci@chromium.org, maruel@chromium.org
BUG=643346
==========