code: def number(self):
          # type: () -> int
          
          cmd = 'git log --oneline {}'.format(self.sha1)
          out = shell.run(cmd, capture=True, never_pretend=True).stdout.strip()
          return len(out.splitlines())
docs: Return this commits number.
  
          This is the same as the total number of commits in history up until
          this commit.
  
          This value can be useful in some CI scenarios as it allows to track
          progress on any given branch (although there can be two commits with the
          same number existing on different branches).
  
          Returns:
              int: The commit number/index.