Try to return as a string the bzr revision of the Emacs sources.
The format is: [revno] revision_id, where revno may be absent.
Value is nil if the sources do not seem to be under bzr, or if we could
not determine the revision. Note that this reports on the current state
of the sources, which may not correspond to the running Emacs.
Optional argument DIR is a directory to use instead of `source-directory'.
Optional argument EXTERNAL non-nil means to maybe ask `bzr' itself,
if the sources appear to be under bzr. If `force', always ask bzr.
Otherwise only ask bzr if we cannot find any information ourselves.