Use 'Auto Merge' for merge commit's base comparison

When reviewing a merge commit, the old wording in the version history dropdown
of 'Base' doesn't really match Gerrit's behavior.  Updating this to use
'Auto Merge' as suggested by SOP on IRC.

Signed-off-by: Brad Larson <bklarson@gmail.com>
Change-Id: Ia71eeb063c1b0d71789dbb584825f4220195ec40
6 files changed