py3: make sure __repr__ returns str

No test fails but I found it while debugging test-commit-interactive-curses.t
failure.

# skip-blame because just r'' prefix

Differential Revision: https://phab.mercurial-scm.org/D5878

mercurial/patch.py

@@ -363,7 +363,7 b' class patchmeta(object):'
         return self._ispatchinga(afile) and self._ispatchingb(bfile)
 
     def __repr__(self):
-        return "<patchmeta %s %r>" % (self.op, self.path)
+        return r"<patchmeta %s %r>" % (self.op, self.path)
 
 def readgitpatch(lr):
     """extract git-style metadata about patches from <patchname>"""