subrepo: fix subrelpath for git subrepos
This patch conforms gitsubrepo code to the variable naming scheme of the other
subrepo implementations. All user-facing path references should be relative
to the root repository.
$ 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