upstreams.

In the event that users are NOT using the enhanced tooling, this is equivalent
to `git merge-base "@{u}" HEAD`. In the event that they are, this will catch
the case where their parent branch got rebased, but this branch hasn't been
rebased on the parent yet.

R=agable@chromium.org, szager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/224703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@261601 0039d316-1c4b-4281-b951-d872f2087c98