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>
diff --git a/project.py b/project.py
index bedc91e..6188ca7 100644
--- a/project.py
+++ b/project.py
@@ -27,7 +27,7 @@
 from error import GitError, ImportError, UploadError
 from error import ManifestInvalidRevisionError
 
-from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB, R_M
+from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB
 
 def _lwrite(path, content):
   lock = '%s.lock' % path
@@ -598,7 +598,7 @@
       return False
 
     if self.worktree:
-      self._InitMRef()
+      self.manifest.SetMRefs(self)
     else:
       self._InitMirrorHead()
       try:
@@ -1080,10 +1080,6 @@
         remote.ResetFetch(mirror=True)
       remote.Save()
 
-  def _InitMRef(self):
-    if self.manifest.branch:
-      self._InitAnyMRef(R_M + self.manifest.branch)
-
   def _InitMirrorHead(self):
     self._InitAnyMRef(HEAD)