git-gui: Let users abort with `reset --hard` type logic.

git-gui: Let users abort with `reset --hard` type logic.

If you get into the middle of a merge that turns out to be horrible
and just not something you want to do right now, odds are you need
to run `git reset --hard` to recover your working directory to a
pre-merge state.

We now offer Merge->Abort Merge for exactly this purpose, however
its also useful to thow away a non-merge, as its basically the same
logic as `git reset --hard`.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>