Add optional file specific log messages

Brain dump from email - it would be nice (several users have asked for this) to be able to provide a log message which has some globally applicable text - i.e.

    *

      Many bug fixes to the UI, particularly the init and branch commands.

and also provide per file messages - i.e.

    *

      foo.c: change the api to init to allow specifying a path
    *

      ui.c: pass a path to init so that the user does not need to chdir

These messages would appear against their filename during bzr log, and when logging only one file, or only one directory, only the matching files would be included in the log output.