re2: byteify some regex used to get check re2 availability

Changeset 687b865b95ad failed to properly byteify this because it was a raw
string. This went undetected so far because re2 does not seems to be widely
tested.

Differential Revision: https://phab.mercurial-scm.org/D10064

mercurial/util.py

@@ -2170,7 +2170,7 b' class _re(object):'
         global _re2
         try:
             # check if match works, see issue3964
-            _re2 = bool(re2.match(r'\[([^\[]+)\]', b'[ui]'))
+            _re2 = bool(re2.match(br'\[([^\[]+)\]', b'[ui]'))
         except ImportError:
             _re2 = False