patch: unify iterhunks() afile/bfile handling git afile/bfile are extracted twice, once when reading a 'diff --git', and again when reading a unified hunk. The problem is not all git blocks have unified hunks (renames just have metadata) and they were not extracted the same way. This is what this patch unifies.

patch: git metadata was ignored if strip > 1 gitpatch objects emitted by iterhunks() are modified in place by applydiff(). Processing them earlier improves iterhunks() isolation. applydiff() modifying them should still be fixed though.