util: stop using readfile() in tempfilter()

util: stop using readfile() in tempfilter()

To unblock code move to utils.*. It's merely two lines of very Pythonic code.
No helper function should be needed.

--- a/mercurial/util.py	Sat Mar 24 14:38:34 2018 +0900
+++ b/mercurial/util.py	Sat Mar 24 14:32:34 2018 +0900
@@ -1528,7 +1528,8 @@
         if code:
             raise error.Abort(_("command '%s' failed: %s") %
                               (cmd, explainexit(code)))
-        return readfile(outname)
+        with open(outname, 'rb') as fp:
+            return fp.read()
     finally:
         try:
             if inname: