This prevents `git rebase-update` from losing work when using a hook like
pre-commit-go.

R=dnj@chromium.org, vadimsh@chromium.org, maruel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1350043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296737 0039d316-1c4b-4281-b951-d872f2087c98