Currently, for a given branch, map-branches displays the number of
commits ahead and behind it is. This change replaces ahead with
commits, representing the number of commits a branch has, starting from
its base commit, and more accurately reflects the state of a repo.

Bug:1128716
Change-Id: I7c070b4efd452d82d878e1cfb7c20d1c80f38ec7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2412991
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>