bdiff: further restrain potential quadratic performance

This causes the longest_match search to limit itself to a window of
30000 lines during search (roughly 1MB), thus avoiding a full O(N*M)
search that might occur in repetitive structured inputs. For a
particular class of many MB pathological test cases, this generated
the following timings:

size    before      after
10x     1.25s       1.24s
100x    57s         33s
1000x   >8400s      400s

The times on the right quickly become much faster and appear more linear.

While windowing means deltas are no longer "optimal", the resulting
deltas were within a couple percent of expected size. While we've yet
to have a report of a file with the level of repetition necessary to
hit this case, some JSON/XML database dump scenario is fairly likely
to hit it.

This may also slightly improve the average-case performance for deltas
of large binaries.

mercurial/bdiff.c

@@ -148,7 +148,15 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 = (a1 + a2) / 2;
+	int mi = a1, mj = b1, mk = 0, mb = 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
+	   reduce skipping overhead on the b chains. */
+	if (a2 - a1 > 30000)
+		a1 = a2 - 30000;
+
+	half = (a1 + a2) / 2;
 
 	for (i = a1; i < a2; i++) {
 		/* skip all lines in b after the current block */