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.

mercurial/patch.py

@@ -1137,8 +1137,6 b' def iterhunks(fp):'
             gp = changed.get(bfile)
             if x.startswith('GIT binary patch'):
                 h = binhunk(gp, lr)
-                afile = 'a/' + afile
-                bfile = 'b/' + bfile
             else:
                 if context is None and x.startswith('***************'):
                     context = True
@@ -1154,15 +1152,16 b' def iterhunks(fp):'
             # check for git diff, scanning the whole patch file if needed
             m = gitre.match(x)
             if m:
-                afile, bfile = m.group(1, 2)
                 if not git:
                     git = True
                     gitpatches = scangitpatch(lr, x)
                     for gp in gitpatches:
-                        changed[gp.path] = gp
+                        changed['b/' + gp.path] = gp
                     yield 'git', gitpatches
+                afile = 'a/' + m.group(1)
+                bfile = 'b/' + m.group(2)
+                gp = changed[bfile]
                 # copy/rename + modify should modify target, not source
-                gp = changed[bfile]
                 if gp.op in ('COPY', 'DELETE', 'RENAME', 'ADD') or gp.mode:
                     afile = bfile
                 newfile = True