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>

subcmds/version.py

@@ -14,11 +14,11 @@
 # limitations under the License.
 
 import sys
-from command import Command
+from command import Command, MirrorSafeCommand
 from git_command import git
 from project import HEAD
 
-class Version(Command):
+class Version(Command, MirrorSafeCommand):
   common = False
   helpSummary = "Display the version of repo"
   helpUsage = """