Add GitcClientCommand class for GITC-specific commands

These won't show up as common commands in the help text unless in a GITC
client, and will refuse to execute.

Change-Id: Iffe82adcc9d6ddde9cb4b204f83ff018042bdab0

command.py

@@ -231,7 +231,12 @@ class MirrorSafeCommand(object):
      and does not require a working directory.
   """
 
-class RequiresGitcCommand(object):
+class GitcAvailableCommand(object):
   """Command that requires GITC to be available, but does
      not require the local client to be a GITC client.
   """
+
+class GitcClientCommand(object):
+  """Command that requires the local client to be a GITC
+     client.
+  """