Make refs/remotes/m management the manifest object's responsibility

I plan to have the new submodule manifest format use a different
layout for the m refs than the XML manifest format has used in
the past.  Thus we need to move the behavior management into the
manifest object, and out of the project, so we can change it.

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

manifest_xml.py

@@ -29,6 +29,7 @@ from error import ManifestParseError
 
 MANIFEST_FILE_NAME = 'manifest.xml'
 LOCAL_MANIFEST_NAME = 'local_manifest.xml'
+R_M = 'refs/remotes/m/'
 
 class _Default(object):
   """Project defaults within the manifest."""
@@ -168,6 +169,10 @@ class XmlManifest(Manifest):
     self._Load()
     return self._default
 
+  def SetMRefs(self, project):
+    if self.branch:
+      project._InitAnyMRef(R_M + self.branch)
+
   def _Unload(self):
     self._loaded = False
     self._projects = {}