Add "repo branch" as an alias for "repo branches"

For those of us that are used to typing "git branch".

Signed-off-by: Mike Lockwood <lockwood@android.com>
(cherry picked from commit 33f0e786bb35ea11da1e3d1746c431da5f1d93eb)

main.py

@@ -61,6 +61,8 @@ class _Repo(object):
   def __init__(self, repodir):
     self.repodir = repodir
     self.commands = all_commands
+    # add 'branch' as an alias for 'branches'
+    all_commands['branch'] = all_commands['branches']
 
   def _Run(self, argv):
     name = None