1. Summarise the issue (what were doing, what went wrong?)

I wanted to recommend that user could upgrade to version x.y.z of darcs,
to use the "new" annotate. To try to do that, I trawled darcs changes &
found mornfall's patch which added the new implementation, but couldn't
easily determine which tag (and thus, version of darcs) contained that
patch.

2. What behaviour were you expecting instead?

I wanted to do something like:
    darcs cha -p 'annotate'
to determine the patch that added new annotate, and then do something like:
    darcs show tags --containing 'the patch in question'
to show me some list of tags, which would be those that transitively
contain the patch, allowing me to say "install version x.y.z or above"
or so.

So, to summarise, currently, `deps :: Tag -> [Patch]` is implemented,
but not `contains :: Patch -> [Tag]`. I suppose this could become a
general purpose matcher, but it doesn't seem to really be
useful/applicable elsewhere.