patch.diff: avoid calling workingctx().manifest()

Right now, to generate the manifest of the working dir, we have to
perform a full walk of the working dir, which will be very slow,
especially if we're interested in only a small part of it.

Since we use the manifest only to find out the mode of files for git
patches, manually build an execf function to do it.

This should fix issue567.

mercurial/patch.py

@@ -495,9 +495,12 b' def diff(repo, node1=None, node2=None, f'
 
     if node2:
         ctx2 = context.changectx(repo, node2)
+        execf2 = ctx2.manifest().execf
     else:
         ctx2 = context.workingctx(repo)
-    man2 = ctx2.manifest()
+        execf2 = util.execfunc(repo.root, None)
+        if execf2 is None:
+            execf2 = ctx2.parents()[0].manifest().copy().execf
 
     # returns False if there was no rename between ctx1 and ctx2
     # returns None if the file was created between ctx1 and ctx2
@@ -563,7 +566,7 b' def diff(repo, node1=None, node2=None, f'
 
             a, b = f, f
             if f in added:
-                mode = gitmode(man2.execf(f))
+                mode = gitmode(execf2(f))
                 if f in copied:
                     a = copied[f]
                     omode = gitmode(man1.execf(a))
@@ -588,7 +591,7 b' def diff(repo, node1=None, node2=None, f'
                     header.append('deleted file mode %s\n' % mode)
             else:
                 omode = gitmode(man1.execf(f))
-                nmode = gitmode(man2.execf(f))
+                nmode = gitmode(execf2(f))
                 addmodehdr(header, omode, nmode)
                 if util.binary(to) or util.binary(tn):
                     dodiff = 'binary'