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

Review-Url: https://codereview.chromium.org/2215673002