This should let the base marker transparently work with plain-old-git tools
which was the idea in the first place. Specifically `git branch -u` without a
corresponding rebase.

R=agable@chromium.org
BUG=373977

Review URL: https://codereview.chromium.org/288323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@271112 0039d316-1c4b-4281-b951-d872f2087c98