Modify git map-branches to show the number of commits on a branch

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>

git_map_branches.py

@@ -248,29 +248,30 @@ class BranchMapper(object):
 
     # The branch tracking status.
     if self.verbosity >= 1:
-      ahead_string = ''
+      commits_string = ''
       behind_string = ''
       front_separator = ''
       center_separator = ''
       back_separator = ''
       if branch_info and not self.__is_invalid_parent(branch_info.upstream):
-        ahead = branch_info.ahead
         behind = branch_info.behind
+        commits = branch_info.commits
 
-        if ahead:
-          ahead_string = 'ahead %d' % ahead
+        if commits:
+          commits_string = '%d commit' % commits
+          commits_string += 's' if commits > 1 else ' '
         if behind:
           behind_string = 'behind %d' % behind
 
-        if ahead or behind:
+        if commits or behind:
           front_separator = '['
           back_separator = ']'
 
-        if ahead and behind:
+        if commits and behind:
           center_separator = '|'
 
       line.append(front_separator, separator=' ')
-      line.append(ahead_string, separator=' ', color=Fore.MAGENTA)
+      line.append(commits_string, separator=' ', color=Fore.MAGENTA)
       line.append(center_separator, separator=' ')
       line.append(behind_string, separator=' ', color=Fore.MAGENTA)
       line.append(back_separator)