Given a patch hash, this command should output details of the patch,
formatted according to a format string or similar.

My original need for this was as a post-processor of annotate
--machine-readable, which gives output in the form:
source_patch_hash line
...

with which, I wanted to do something like:

$ darcs annotate --machine foo.hs | while read hash; do darcs show patch
-f '%e' $hash; done | sort | uniq -c

to extract an ordered list of authors, on a given file.

owst, could you post a more detailed "screenshot" mockup of what the tool 
would look like in action?

Something like this:

    > darcs show patch --format '%an %d %n' $hash
    Owen Stephens<TAB>01/01/2012 00:00:00+00:00<TAB>This is my patch name

expected format specifiers:
    %a - author
    %an - author name
    %ae - author email
    %d - date
    %n - name
    %l - log
    %s - summary
    %c - changes
    ...

I suppose we should try and be consistent with what other VCSs provide
in their
format specifiers.

<TAB> could reasonably be \0 also.

I think that's what I would like it to look like.