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.

pulling from ../a
searching for changes
abort: repository is unrelated
pulling from ../a
searching for changes
warning: repository is unrelated
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
changeset:   1:bdcee5d51fa6
tag:         tip
user:        a
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     a

changeset:   0:f155ba1aa5ba
user:        b
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     b