Fix to mirror manifest when --mirror is given

Commit 8d201 "repo: Support multiple branches for the same project."
(Change id is I5e2f4e1a7abb56f9d3f310fa6fd0c17019330ecd) caused missing
mirroring manifest repository when 'repo sync' after 'repo init --mirror'.

When the function _AddMetaProjectMirror() is called to add two of
meta projects - git-repo itself and manifest repository to mirror,
it didn't add them into self._paths which has list of projects to be
sync'ed by 'repo sync'.

In addition, because member var of Project 'relpath' is used as a key
of self._paths, it should be set with proper value other than None.
Since this is only for meta projects which are not described in manifest
xml, 'relpath' is name of the projects.

Change-Id: Icc3b9e6739a78114ec70bf54fe645f79df972686
Signed-off-by: Kwanhong Lee <kwanhong.lee@windriver.com>

manifest_xml.py

@@ -568,10 +568,11 @@ class XmlManifest(object):
                         gitdir = gitdir,
                         objdir = gitdir,
                         worktree = None,
-                        relpath = None,
+                        relpath = name or None,
                         revisionExpr = m.revisionExpr,
                         revisionId = None)
       self._projects[project.name] = [project]
+      self._paths[project.relpath] = project
 
   def _ParseRemote(self, node):
     """