Allow user to specify git path for mirror repositories.

In some case, especially local_manifest.xml for mirror repos,
user may want to specify a different gitdir path.

The project's 'path' attr must be set in the mirror manifest files,
meanwhile, its 'force-path' attr must be set to 'true'.

Change-Id: Ib0a639e9014811701fbc4eccc3b03330d9b9f4b2
Signed-off-by: Scott Fan <fancp2007@gmail.com>