subrepo: use posixpath when diffing, for consistent paths

This fixes a Windows failure in test-subrepo-recursion.t introduced
by c84ef0047a94.

mercurial/patch.py

@@ -6,7 +6,7 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
-import cStringIO, email.Parser, os, errno, re
+import cStringIO, email.Parser, os, errno, re, posixpath
 import tempfile, zlib, shutil
 
 from i18n import _
@@ -1655,7 +1655,7 b' def trydiff(repo, revs, ctx1, ctx2, modi'
             copy, getfilectx, opts, losedatafn, prefix):
 
     def join(f):
-        return os.path.join(prefix, f)
+        return posixpath.join(prefix, f)
 
     def addmodehdr(header, omode, nmode):
         if omode != nmode:

mercurial/subrepo.py

@@ -446,7 +446,7 b' class hgsubrepo(abstractsubrepo):'
                 node2 = node.bin(node2)
             cmdutil.diffordiffstat(self._repo.ui, self._repo, diffopts,
                                    node1, node2, match,
-                                   prefix=os.path.join(prefix, self._path),
+                                   prefix=posixpath.join(prefix, self._path),
                                    listsubrepos=True, **opts)
         except error.RepoLookupError, inst:
             self._repo.ui.warn(_('warning: error "%s" in subrepository "%s"\n')