Limit Bazaar logs to one fewer revision with revision ranges.

When guessing the summary and description for Bazaar using revision ranges, the
guessed results would include the commit message for the orig revision of the
diff, which isn't right. This change does one log operation to get the number
of revisions, and then another one, limiting the results to that number less
one.

Testing done: Ran unit tests.

Fixes bug 2959.

Reviewed at https://reviews.reviewboard.org/r/5491/