Don't permit users to run repo status in a mirror client

If a client was created with "repo init --mirror" then there are
no working directories present, and no files checked out.  Using
a command like "repo status" in this context makes no sense, and
actually throws back a Pytyon traceback at the console when the
underlying commands fail out.

We now tag commands with the MirrorSafeCommand type if they are
able to be executed within a mirror directory safely.  Using a
command in a mirror which lacks this base class results in a
useful error letting you know the command isn't supported.

Bug: REPO-14
Signed-off-by: Shawn O. Pearce <sop@google.com>

main.py

@@ -27,7 +27,9 @@ import os
 import re
 import sys
 
-from command import InteractiveCommand, PagedCommand
+from command import InteractiveCommand
+from command import MirrorSafeCommand
+from command import PagedCommand
 from editor import Editor
 from error import ManifestInvalidRevisionError
 from error import NoSuchProjectError
@@ -91,6 +93,12 @@ class _Repo(object):
     cmd.manifest = Manifest(cmd.repodir)
     Editor.globalConfig = cmd.manifest.globalConfig
 
+    if not isinstance(cmd, MirrorSafeCommand) and cmd.manifest.IsMirror:
+      print >>sys.stderr, \
+            "fatal: '%s' requires a working directory"\
+            % name
+      sys.exit(1)
+
     if not gopts.no_pager and not isinstance(cmd, InteractiveCommand):
       config = cmd.manifest.globalConfig
       if gopts.pager: