"bzr describe" command

[Jelmer Vernooij]

 affects bzr
 status confirmed
 importance wishlist

Git has a "git describe" command that can generate a string based on the
"latest tag" (the tag found on the most recent tagged revision in the
current branch) and the current revision string.

bzr-builddeb has something similar to this, to generate debian version
strings from arbitrary revisions.

I'd like to move most of this logic from bzr-builddeb to bzrlib and
expose it to API and command-line users.

I'm not entirely sure whether another top-level command is appropriate.
Perhaps it could be a separate option to "bzr version-info" or "bzr
revision-info" ?