context: rename local 'attr' to 'attr_'

In the next diff we're going to import mercurial.thirdparty.attr, and pyflakes
complains about this if this rename isn't done.

Differential Revision: https://phab.mercurial-scm.org/D897

mercurial/context.py

@@ -2420,9 +2420,9 b' class overlayfilectx(committablefilectx)'
         if reusable:
             # copy extra fields from originalfctx
             attrs = ['rawdata', 'rawflags', '_filenode', '_filerev']
-            for attr in attrs:
-                if util.safehasattr(originalfctx, attr):
-                    setattr(self, attr, getattr(originalfctx, attr))
+            for attr_ in attrs:
+                if util.safehasattr(originalfctx, attr_):
+                    setattr(self, attr_, getattr(originalfctx, attr_))
 
     def data(self):
         return self._datafunc()