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.
basic test for hg debugrebuildstate
$ hg init repo
$ cd repo
$ touch foo bar
$ hg ci -Am 'add foo bar'
adding bar
adding foo
$ touch baz
$ hg add baz
$ hg rm bar
$ hg debugrebuildstate
state dump after
$ hg debugstate --nodates | sort
n 666 -1 bar
n 666 -1 foo
status
$ hg st -A
! bar
? baz
C foo