gclient_scm: Make calls to "git merge-base" non-fatal.

gclient_scm: Make calls to "git merge-base" non-fatal.

It might be the case for some repositories that there is no merge base
between the current git HEAD and origin/master, which causes git
merge-base to exit with a non-zero code and cause calls to gclient
status/diff/pack to fail, as well as gclient sync if the repository in
question has been removed from DEPS.

This is true for the external/webrtc/trunk/talk repository, for example.
Its recent release branches (branch-heads/45 all the way to /53 at
least) have no ancestry shared with its master branch, so gclient
sync'ing from a Chromium M51 checkout to an M52 one (where it's no
longer in DEPS) fails because of the failed git merge-base calls.

We now ignore failures and just don't specify a merge base when calling
"git diff".

BUG=633962
R=iannucci@chromium.org,agable@chromium.org,maruel@chromium.org

Committed: https://chromium.googlesource.com/chromium/tools/depot_tools/+/05c835919467e87b13c39c687cdbe659300784ea

[Raphael Kubo da Costa (rakuco), 2016-08-04 09:36:01]

PTAL.

This is the past of least effort to fixing/working around the gclient failures I
mentioned here and in the related bug report.

I'm not entirely convinced we need the calls to merge-base, to be honest: it
seems to be used as a way of telling the calls to git diff to return all changes
since the current branch was created from master, which does not make much sense
on release branches or any branch not created from master. I think the code in
GitWrapper.update() does a better job at finding what to compare the current
HEAD against, but the logic there is quite convoluted and moving the code around
will likely break something.

[M-A Ruel, 2016-08-04 15:06:28]

lgtm

[Raphael Kubo da Costa (rakuco), 2016-08-04 15:29:03]

The CQ bit was checked by raphael.kubo.da.costa@intel.com

[commit-bot: I haz the power, 2016-08-04 15:29:13]

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, 2016-08-04 15:32:45]

Description was changed from

==========
gclient_scm: Make calls to "git merge-base" non-fatal.

It might be the case for some repositories that there is no merge base
between the current git HEAD and origin/master, which causes git
merge-base to exit with a non-zero code and cause calls to gclient
status/diff/pack to fail, as well as gclient sync if the repository in
question has been removed from DEPS.

This is true for the external/webrtc/trunk/talk repository, for example.
Its recent release branches (branch-heads/45 all the way to /53 at
least) have no ancestry shared with its master branch, so gclient
sync'ing from a Chromium M51 checkout to an M52 one (where it's no
longer in DEPS) fails because of the failed git merge-base calls.

We now ignore failures and just don't specify a merge base when calling
"git diff".

BUG=633962
R=iannucci@chromium.org,agable@chromium.org,maruel@chromium.org
==========

to

==========
gclient_scm: Make calls to "git merge-base" non-fatal.

It might be the case for some repositories that there is no merge base
between the current git HEAD and origin/master, which causes git
merge-base to exit with a non-zero code and cause calls to gclient
status/diff/pack to fail, as well as gclient sync if the repository in
question has been removed from DEPS.

This is true for the external/webrtc/trunk/talk repository, for example.
Its recent release branches (branch-heads/45 all the way to /53 at
least) have no ancestry shared with its master branch, so gclient
sync'ing from a Chromium M51 checkout to an M52 one (where it's no
longer in DEPS) fails because of the failed git merge-base calls.

We now ignore failures and just don't specify a merge base when calling
"git diff".

BUG=633962
R=iannucci@chromium.org,agable@chromium.org,maruel@chromium.org

Committed:
https://chromium.googlesource.com/chromium/tools/depot_tools/+/05c835919467e8...
==========

[commit-bot: I haz the power, 2016-08-04 15:32:45]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/tools/depot_tools/+/05c835919467e8...