Refactor Manifest to be XmlManifest

We'll soon be supporting two different manifest formats, but we
can't immediately remove support for the current XML one that is
in wide spread use within Android.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/repo b/repo
index 176c563..1374255 100755
--- a/repo
+++ b/repo
@@ -505,7 +505,7 @@
   my_git = os.path.join(my_dir, '.git')
 
   if os.path.isfile(my_main) and os.path.isdir(my_git):
-    for name in ['manifest.py',
+    for name in ['git_config.py',
                  'project.py',
                  'subcmds']:
       if not os.path.exists(os.path.join(my_dir, name)):