Motivation is something like:

I've cloned a remote repo, hacked locally and then `darcs send`d my
patches. 

Now, I'd like to record some new patches without them depending on those
other patches I sent. 

I'd like to be able to say `darcs obliterate --notInRemote remote_path`
to easily obliterate those patches that aren't available in the remote repo.

Also, Florent is going to buy me a beer, if I implement this feature ;-)

The following patch sent by Owen Stephens <darcs@owenstephens.co.uk> updated issue issue2225 with
status=resolved;resolvedin=2.10.0 HEAD

* resolve issue2225: add an option to obliterate all patches not in remote repo 
Ignore-this: d0f75deb5d8090f84968d0018afe6b5e