git-gui: Improve the merge check interface for branch deletion.

git-gui: Improve the merge check interface for branch deletion.

Just like how we split out the local and remote branches into two
different pick lists for branch creation, we should do the same
thing for branch deletion.  This means that  there are really 3
modes of operation here:

  * delete only if merged into designated local branch;
  * delete only if merged into designated tracking (remote) branch;
  * delete no matter what

So we now use radio buttons to select between these operations.

We still default to checking for merge into the current branch,
as that is probably the most commonly used behavior. It also is
what core Git's command line tools do.

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