bdiff: improve worst case behavior by 100x. on 5.8MB (244.000 lines) text file with similar lines, hash before this change made diff against empty file take 75 seconds. this change improves performance to 0.6 seconds. result is that clone of smallish repo (137MB) with some files like this takes 1 minute instead of 10 minutes. common case of diff is 10% slower now, probably because of worse cache locality. but diff does not affect overall performance in common case (less than 1% of runtime is in diff when it is working ok), so this tradeoff looks good.

(the addremove command is deprecated; use add and remove --after instead)
adding dir/bar
adding foo
dir/bar
foo
(the addremove command is deprecated; use add and remove --after instead)
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2