Creating rr-cache

If git-rerere is enabled, it uses the rr-cache directory that
repo currently creates a symlink from, but doesn't create the
destination directory (inside the project's directory). Git
will then complain during merges and rebases.

This commit creates the rr-cache directory inside the project.

Change-Id: If8b57a04f022fc6ed6a7007d05aa2e876e6611ee

project.py

@@ -1547,6 +1547,11 @@ class Project(object):
       cmd.append(HEAD)
       if GitCommand(self, cmd).Wait() != 0:
         raise GitError("cannot initialize work tree")
+
+      rr_cache = os.path.join(self.gitdir, 'rr-cache')
+      if not os.path.exists(rr_cache):
+        os.makedirs(rr_cache)
+
       self._CopyFiles()
 
   def _gitdir_path(self, path):