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.

mercurial/diffhelpers.py

@@ -69,7 +69,7 b' 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]: