git-gui: Implemented local branch deletion.

git-gui: Implemented local branch deletion.

Users can now delete a local branch by selecting from a list of
available branches.  The list automatically does not include
the current branch, as deleting the current branch could be quite
dangerous and should not be supported.

The user may also chose to have us verify the branches are fully
merged into another branch before deleting them.  By default we
select the current branch, matching 'git branch -d' behavior,
but the user could also select any other local branch.

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