[PATCH 2 of 2 v2] match: use re2 in readpatternfile if possible

[Bryan O'Sullivan]

# HG changeset patch
# User Bryan O'Sullivan <bos at serpentine.com>
# Date 1449812035 28800
#      Thu Dec 10 21:33:55 2015 -0800
# Node ID 18f300e9ca45cdae3575d1860251929abf07b42c
# Parent  dea2f8f8a033c13083fe04609bf014b2f58dfcf3
match: use re2 in readpatternfile if possible

This has a small, but measurable, effect on performance if a pattern
file is very large.  In an artificial test with 200,000 lines of
pattern data, using re2 reduced read time by 200 milliseconds.

diff --git a/mercurial/match.py b/mercurial/match.py
--- a/mercurial/match.py
+++ b/mercurial/match.py
@@ -654,9 +654,11 @@ def readpatternfile(filepath, warn):
         if "#" in line:
             global _commentre
             if not _commentre:
-                _commentre = re.compile(r'((^|[^\\])(\\\\)*)#.*')
+                _commentre = util.re.compile(r'((?:^|[^\\])(?:\\\\)*)#.*')
             # remove comments prefixed by an even number of escapes
-            line = _commentre.sub(r'\1', line)
+            m = _commentre.search(line)
+            if m:
+                line = line[:m.end(1)]
             # fixup properly escaped comments that survived the above
             line = line.replace("\\#", "#")
         line = line.rstrip()