Print '(no branches)' if the output of repo branches is empty

This way its clear the command did something, and reported
that it had nothing to show you, because you have no active
branches in this client.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/branches.py

@@ -96,6 +96,10 @@ Summarizes the currently available topic branches.
           n.append(name)
       names = n
 
+    if not names:
+      print >>sys.stderr, '   (no branches)'
+      return
+
     width = 25
     for name in names:
       if width < len(name):