bdiff: do not use recursion / avoid stackoverflow (issue1940)

mercurial/bdiff.c

@@ -226,6 +226,7 b' static void recurse(struct line *a, stru'
 {
 	int i, j, k;
 
+	while (1) {
 	/* find the longest match in this chunk */
 	k = longest_match(a, b, pos, a1, a2, b1, b2, &i, &j);
 	if (!k)
@@ -238,7 +239,10 b' static void recurse(struct line *a, stru'
 	l->head->b1 = j;
 	l->head->b2 = j + k;
 	l->head++;
-	recurse(a, b, pos, i + k, a2, j + k, b2, l);
+		/* tail-recursion didn't happen, so doing equivalent iteration */
+		a1 = i + k;
+		b1 = j + k;
+	}
 }
 
 static struct hunklist diff(struct line *a, int an, struct line *b, int bn)