Some SVN-only trybots get errors when running gclient revert with this
change. I assume it's because the flag is not present for the gclient
revert command, and that the code checking if options.with_branch_heads
is set is common to both revert and sync.

BUG=none
TEST=reverted locally and gclient revert worked again.

Review URL: https://chromiumcodereview.appspot.com/12616006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@186967 0039d316-1c4b-4281-b951-d872f2087c98