support bzr per-file comments

Bazaar supports per-file comments in revisions - using bzr-gtk one can
write a global revisions comment and comments for individual files too
(this was apparently modeled after a similar feature in BitKeeper).
When exporting a bzr repository to git these per-file comments are lost.

This patch makes them to be not lost but appear as a part of the commit
comment instead. Empty file comments and file comments identical to the
commit comment are skipped (they originate from tools and policies that
require writing per-file comments and users simply copy-paste revision
comment over, these comments add no value as a part of the commit comment).
Corrupted file comments are ignored too (there is one old - 2008 - revision
corrupted like that in the MariaDB/MySQL tree).