context: add cmp for filectxs

mercurial/context.py

@@ -193,6 +193,8 b' class filectx(object):'
     def path(self): return self._path
     def size(self): return self._filelog.size(self._filerev)
 
+    def cmp(self, text): return self._filelog.cmp(self._filenode, text)
+
     def parents(self):
         p = self._path
         fl = self._filelog
@@ -467,3 +469,5 b' class workingfilectx(filectx):'
         return []
 
     def size(self): return os.stat(self._repo.wjoin(self._path)).st_size
+
+    def cmp(self, text): return self._repo.wread(self._path) == text