Before, for the common case that the actual_merge_base was already set
as base, the check for is_ancestor(base, actual_merge_base) would have
caused the base to be re-set.
This is a writing operation, which causes trouble e.g. for the parallel
operation of 'git cl status'.

R=iannucci@chromium.org, tandrii@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1888213004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300003 0039d316-1c4b-4281-b951-d872f2087c98