Added remote 'git' branch awareness to 'gclient'

Added remote 'git' branch awareness to 'gclient'; this is needed to check out specific
Gerrit patch sets, as those aren't included with the standard 'master' pull.
This is done by adding a common revision parsing function and adding
additional 'fetch' commands at appropriate points.

Added test for revision parsing and related functionality.

BUG=278188
TEST=local,unittests
R=agable@chromium.org, maruel@chromium.org

[dnj (Google), 2014-04-24 19:30:44]

Hey, could you please review and/or weigh in on some proposed 'gclient' changes?
The motivation for these changes is to enable 'gclient' to check out remote refs
that aren't part of the remote's 'fetch' section by default. Specifically,
Gerrit patch sets cannot be used as revisions with 'gclient'.

This would allow one to fetch a URL such as:
https://chromium.googlesource.com/chromiumos/chromite@refs/changes/30/196630/1

I took the approach of having 'gclient' analyze the revision and manually create
a local ref for any remote ref that's not part of the (assumed) default fetch
spec. Thus, this change would cause 'gclient' to run:

git fetch refs/changes/30/196630/1 refs/changes/30/196630/1

... causing the subsequent 'git checkout refs/changes/30/196630/1' to work.

An alternative option would be to allow the '.gclient' file to be augmented with
a 'fetch' list that specified remote refs to fetch in addition to the default
refs, but this felt more manual and after a conversation (a while back) with
agable I opted for the current implementation.

Thanks!

https://codereview.chromium.org/250523004/diff/1/gclient_scm.py
File gclient_scm.py (right):

https://codereview.chromium.org/250523004/diff/1/gclient_scm.py#newcode865
gclient_scm.py:865: if os.path.exists(tmp_dir) and os.listdir(tmp_dir):
A bit unrelated, but I did hit this case when 'listdir' was called and the
directory was not created.

[M-A Ruel, 2015-01-15 18:53:08]

I just saw this CL, is it still applicable?

[dnj, 2015-01-15 18:55:17]

Probably not. I'll close it, as it'll almost certainly need to be recreated even
if used.