Fix bzr plugin to work with bzr 2.0.

The output of "bzr log" seems to have changed a bit, so we change the
parsing accordingly. This has not been tested with earlier versions of
bzr.

Several problems seemed to occur, all in the bzr_log subroutine:

1. The @infos list would contain an empty hash, which would confuse the
   rest of the program.
2. This was because bzr_log would push an empty anonymous hash to the
   list whenever it thought a new record would start.
3. However, a new record marker (now?) also happens at th end of bzr log
   output.
4. Now we collect the record to a hash that gets pushed to the list only
   if it is not empty.
5. Also, sometimes bzr log outputs "revno: 1234 [merge]", so we catch only
   the revision number.
6. Finally, there may be non-headers at the of the output, so we ignore
   those.