lock: take both vfs and lock file path relative to vfs to access via vfs
This patch makes "lock.lock.__init__()" take both vfs and lock file
path relative to vfs, instead of absolute path to lock file.
This allows lock file to be accessed via vfs.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state