diff: make `hg diff --root=subrepo` work The root prefix is currently stripped before adding the subrepo prefix. It seems to me that if you run `hg diff --root=subrepo` and a subrepo/ path is visited (say "subrepo/foo"), then the path generated for the diff would initially be just "foo" and we then fail (if the developer warning is active) to strip the "subrepo/" from the path. It seems like we should first add the prefix in order to produce a path that's relative to the top-level repo, and then we can remove the root prefix from that. There are no tests for this, it seems, and I don't care enough to add one. Differential Revision: https://phab.mercurial-scm.org/D5895