# HG changeset patch
# User Yuya Nishihara <yuya@tcha.org>
# Date 2018-04-19 14:33:17
# Node ID 090c89a8db32399adec8778f7c62fdf70e86c52b
# Parent  5cab6f5016fa5e2c50f89bbfc2f92369efd41522

diffhelpers: backport 9e40bc4c1bde from C implementation

9e40bc4c1bde just says "harden testhunk." I don't think this would be
the case, but it makes some sense to avoid negative index.

diff --git a/mercurial/diffhelpers.py b/mercurial/diffhelpers.py
--- a/mercurial/diffhelpers.py
+++ b/mercurial/diffhelpers.py
@@ -69,7 +69,7 @@ def testhunk(a, b, bstart):
     """
     alen = len(a)
     blen = len(b)
-    if alen > blen - bstart:
+    if alen > blen - bstart or bstart < 0:
         return False
     for i in xrange(alen):
         if a[i][1:] != b[i + bstart]: