contrib/clang-format-blacklist

--- a/contrib/clang-format-blacklist	Mon Feb 12 10:29:02 2018 -0500
+++ b/contrib/clang-format-blacklist	Mon Feb 12 10:31:17 2018 -0500
@@ -3,7 +3,6 @@
 mercurial/cext/base85.c
 mercurial/cext/charencode.c
 mercurial/cext/charencode.h
-mercurial/cext/diffhelpers.c
 mercurial/cext/dirs.c
 mercurial/cext/manifest.c
 mercurial/cext/mpatch.c