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/main.py b/main.py
index 774b903..70ddeff 100755
--- a/main.py
+++ b/main.py
@@ -36,7 +36,7 @@
 from error import ManifestInvalidRevisionError
 from error import NoSuchProjectError
 from error import RepoChangedException
-from manifest import Manifest
+from manifest_xml import XmlManifest
 from pager import RunPager
 
 from subcmds import all as all_commands
@@ -97,7 +97,7 @@
       sys.exit(1)
 
     cmd.repodir = self.repodir
-    cmd.manifest = Manifest(cmd.repodir)
+    cmd.manifest = XmlManifest(cmd.repodir)
     Editor.globalConfig = cmd.manifest.globalConfig
 
     if not isinstance(cmd, MirrorSafeCommand) and cmd.manifest.IsMirror: