[PATCH 03 of 10] patch: stop using cext.diffhelpers

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1523274579 -32400
#      Mon Apr 09 20:49:39 2018 +0900
# Node ID 2d521734ec51a8dbf99c77fd0c2242669c7b5109
# Parent  35a82b6cd55528b0aa0028321c69756d41175b54
patch: stop using cext.diffhelpers

The C implementation has a couple of memory bugs, and lacks error handling
which could lead to SEGV. I could fix them one by one (and I mostly finished
that), but the performance gain provided by cext.diffhelper is quite low.
Besides, diffhelpers.addlines() calls back Python, linereader.readline(),
from the innermost loop.

  $ hg export -R mozilla-central 0:100 > patch
  $ ls -lh patch
  -rw-r--r--  184M  patch
  $ hg init repo && hg -R repo import patch --time --bypass
  (cext) time: real 34.970 secs (user 32.720+0.000 sys 2.230+0.000)
  (pure) time: real 35.950 secs (user 33.600+0.000 sys 2.330+0.000)

So, let's simply use the pure Python implementation.

diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -33,20 +33,19 @@ from . import (
     mail,
     mdiff,
     pathutil,
-    policy,
     pycompat,
     scmutil,
     similar,
     util,
     vfs as vfsmod,
 )
+from .pure import diffhelpers
 from .utils import (
     dateutil,
     procutil,
     stringutil,
 )
 
-diffhelpers = policy.importmod(r'diffhelpers')
 stringio = util.stringio
 
 gitre = re.compile(br'diff --git a/(.*) b/(.*)')