This adds the '--auto_rebase' sync option, enabling parallel updates to
automatically rebase local Git branches during sync.

BUG=None
TEST=local

Review URL: https://codereview.chromium.org/930693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294151 0039d316-1c4b-4281-b951-d872f2087c98