8.1.5 release note

As usual, following item in the 8.1.5 release note is pretty vague:
    * Efficiency improvements in hash tables and bitmap index scans(Tom)

Especially I'm wondering what was actually improved in bitmap index
scans. I see several commit messages regarding bitmap index scans, but
I cannot figure out which one is related to the item.

Any idea?
--
Tatsuo Ishii
SRA OSS, Inc. Japan