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

I wanted to rollback part of a patch under a tag, but darcs won't
present the patch to me if I select the patch using -p 'patch name' and
the name of the affected file, e.g.

darcs roll -p 'patch name' pathToFile

2. What behaviour were you expecting instead?

To be able to pick out hunks from changes under a tag.

3. What darcs version are you using? (Try: darcs --exact-version)

2.9.4 (+ 11 patches)

4. What operating system are you running?

Linux x86_64

This is really a UI problem: tags can't be rolled-back, yet they can
prevent patches being presented, since they depend on everything not
depended on (and you can't rollback a depended on patch if you don't
rollback the dependencies).

My proposal is to somehow "ignore" tags and their (implicit)
dependencies when presenting the list of changes to the user.

I ran into this UI issue today as well. I wanted to rollback a patch 
beneath several tags, and it appeared at first I couldn't without 
"rolling back the patches". But it turned out to be a UI issue.  I had to 
say "Yes" to rolling back each patch, even though nothing actually 
happened with them.

There's a more complicated case too: suppose you have a small patch A 
that depends on a piece of a large patch B, and you want to rollback 
some unrelated part of B. You still need to say 'y' to A to be offered 
B.

Perhaps we should offer any patches which have some parts that could be 
rolled back given the current choices. And/or have 'w' in patch 
selection mean "I don't want to roll this patch itself back but offer 
any bits that are dependencies of things I do want to roll back".