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.
http://mercurial.selenic.com/bts/issue433
$ hg init
$ echo a > a
$ hg commit -Ama
adding a
$ hg parents -r 0 doesnotexist
abort: 'doesnotexist' not found in manifest!
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