--- a/mercurial/diffhelpers.py	Fri Apr 20 20:38:29 2018 +0900
+++ b/mercurial/diffhelpers.py	Thu Apr 19 23:33:17 2018 +0900
@@ -69,7 +69,7 @@
     """
     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]: