workingfilectx.cmp: invert boolean return value

Apparently we mostly used filectx.cmp(workingfilectx.read()), so no error
was ever triggered, but since all cmp() methods return True when content
are different, that == should in fact be !=

mercurial/context.py

@@ -932,7 +932,7 b' class workingfilectx(filectx):'
             return (t, tz)
 
     def cmp(self, text):
-        return self._repo.wread(self._path) == text
+        return self._repo.wread(self._path) != text
 
 class memctx(object):
     """Use memctx to perform in-memory commits via localrepo.commitctx().