git-gui: Delete branches with 'git branch -D' to clear config

git-gui: Delete branches with 'git branch -D' to clear config

If we are deleting a local branch from refs/heads/ we need to
make sure any associated configuration stored in .git/config is
also removed (such as branch.$name.remote and branch.$name.merge).
The easiest way to do this is to use git-branch as that automatically
will look for and delete configuration keys as necessary.

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