Don't permit "repo init --mirror" in an existing client

Simply setting repo.mirror true doesn't make a client into a mirror.
The on-disk layout is completely wrong for a mirror repository,
and until we fix our layout for a non-mirror client to more closely
resemble the upstream we can't do anything to easily turn on or
turn off the mirror status flag.

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

subcmds/init.py

@@ -89,8 +89,9 @@ default.xml will be used.
 
   def _SyncManifest(self, opt):
     m = self.manifest.manifestProject
+    is_new = not m.Exists
 
-    if not m.Exists:
+    if is_new:
       if not opt.manifest_url:
         print >>sys.stderr, 'fatal: manifest url (-u) is required.'
         sys.exit(1)
@@ -117,7 +118,11 @@ default.xml will be used.
       r.Save()
 
     if opt.mirror:
-      m.config.SetString('repo.mirror', 'true')
+      if is_new:
+        m.config.SetString('repo.mirror', 'true')
+      else:
+        print >>sys.stderr, 'fatal: --mirror not supported on existing client'
+        sys.exit(1)
 
     m.Sync_NetworkHalf()
     m.Sync_LocalHalf()