[MERGE][0.16] Improvements to "bzr log file"

[John Arbash Meinel]

The attached patch brings in Kent Gibson's changes to "bzr log file" to
provide correct behavior for merged revisions, and to improve
performance dramatically when dealing with large revision ranges.

This fixes a couple long-standing bugs.

The new function is moderately well tested, so I'm happy enough with it.

I'd really like to see this merged for 0.16, so if we can get a review,
I'll be happy to do a quick turnaround to address any comments.

John
=:->