bdiff: remove effectively dead code

Now that we extend matches backwards in the inner loop, the final
adjustment has no effect.

(A similar extension for the forward direction is trickier and has
less benefit.)

mercurial/bdiff.c

@@ -148,7 +148,7 b' static int equatelines(struct line *a, i'
 static int longest_match(struct line *a, struct line *b, struct pos *pos,
 			 int a1, int a2, int b1, int b2, int *omi, int *omj)
 {
-	int mi = a1, mj = b1, mk = 0, mb = 0, i, j, k, half;
+	int mi = a1, mj = b1, mk = 0, i, j, k, half;
 
 	/* window our search on large regions to better bound
 	   worst-case performance. by choosing a window at the end, we
@@ -195,18 +195,15 b' static int longest_match(struct line *a,'
 		mj = mj - mk + 1;
 	}
 
-	/* expand match to include neighboring popular lines */
-	while (mi - mb > a1 && mj - mb > b1 &&
-	       a[mi - mb - 1].e == b[mj - mb - 1].e)
-		mb++;
+	/* expand match to include subsequent popular lines */
 	while (mi + mk < a2 && mj + mk < b2 &&
 	       a[mi + mk].e == b[mj + mk].e)
 		mk++;
 
-	*omi = mi - mb;
-	*omj = mj - mb;
+	*omi = mi;
+	*omj = mj;
 
-	return mk + mb;
+	return mk;
 }
 
 static struct hunk *recurse(struct line *a, struct line *b, struct pos *pos,