Optimize the checksum code for the ChangeSet file. It is good for another 40% speedup on 'bk checksum' and helps takepatch as well.
Optimize the checksum code for the ChangeSet file.
It is good for another 40% speedup on 'bk checksum' and
helps takepatch as well.

bk: 40e07507jA3-fth1ml7Wktxs2iXrwQ
bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper