Fix another sortdiff cornercase

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Hash: SHA1

Fix another sortdiff cornercase

manifest hash: 32b2d1750689c561c9471fb68a9f3571364399cb
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mercurial/mdiff.py

@@ -58,14 +58,14 b' def sortdiff(a, b):'
             yield (am, bm, la - am) # return a match
 
         # skip mismatched lines from b
-        while lb < lenb and b[lb] < a[la]:
+        while la < lena and lb < lenb and b[lb] < a[la]:
             lb += 1
 
         if lb >= lenb:
             break
         
         # skip mismatched lines from a
-        while la < lena and b[lb] > a[la]:
+        while la < lena and lb < lenb and b[lb] > a[la]:
             la += 1
 
         if la >= lena: