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

I did a send -O on a WIP patch, when I already had wip.dpatch in the
directory. The existing file was overwritten without warning.

2. What behaviour were you expecting instead?

Darcs should rename the file it outputs, rather than overwriting my file.

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

2.9.7

This is slightly tricky to fix.

There's code that does a similar thing in UI/Commands/Unrecord.hs, and
in Util.hs in the same directory, but I suspect the better place would
be in getOuput in ../Arguments.hs, where it can be made to work for all
dpatch-emitting commands, rather than in an ad-hoc fashion.

I should be able to get this done with a bit more thought.