patch: fix patchmeta/hunk synchronization in iterhunks()

Synchronizing on bfile does not work on file removal where bfile is /dev/null.
We match items on afile or bfile instead. The incorrect code makes iterhunks()
to emit patchmeta and hunks separately in some cases. This is currently hidden
by applydiff() being too tolerant when processing patchmeta, and will be fixed
later.

mercurial/patch.py

@@ -1122,8 +1122,9 b' def iterhunks(fp):'
             or (context is not False and x.startswith('***************'))
             or x.startswith('GIT binary patch')):
             gp = None
-            if gitpatches and gitpatches[-1][0] == bfile:
-                gp = gitpatches.pop()[1]
+            if (gitpatches and
+                (gitpatches[-1][0] == afile or gitpatches[-1][1] == bfile)):
+                gp = gitpatches.pop()[2]
             if x.startswith('GIT binary patch'):
                 h = binhunk(lr)
             else:
@@ -1141,17 +1142,17 b' def iterhunks(fp):'
                 continue
             if gitpatches is None:
                 # scan whole input for git metadata
-                gitpatches = [('b/' + gp.path, gp) for gp
+                gitpatches = [('a/' + gp.path, 'b/' + gp.path, gp) for gp
                               in scangitpatch(lr, x)]
-                yield 'git', [g[1] for g in gitpatches
-                              if g[1].op in ('COPY', 'RENAME')]
+                yield 'git', [g[2] for g in gitpatches
+                              if g[2].op in ('COPY', 'RENAME')]
                 gitpatches.reverse()
             afile = 'a/' + m.group(1)
             bfile = 'b/' + m.group(2)
-            while bfile != gitpatches[-1][0]:
-                gp = gitpatches.pop()[1]
+            while afile != gitpatches[-1][0] and bfile != gitpatches[-1][1]:
+                gp = gitpatches.pop()[2]
                 yield 'file', ('a/' + gp.path, 'b/' + gp.path, None, gp)
-            gp = gitpatches[-1][1]
+            gp = gitpatches[-1][2]
             # copy/rename + modify should modify target, not source
             if gp.op in ('COPY', 'DELETE', 'RENAME', 'ADD') or gp.mode:
                 afile = bfile
@@ -1189,7 +1190,7 b' def iterhunks(fp):'
             hunknum = 0
 
     while gitpatches:
-        gp = gitpatches.pop()[1]
+        gp = gitpatches.pop()[2]
         yield 'file', ('a/' + gp.path, 'b/' + gp.path, None, gp)
 
 def applydiff(ui, fp, changed, backend, store, strip=1, eolmode='strict'):