git-gui: always default to the last merged branch in remote delete

git-gui: always default to the last merged branch in remote delete

This is useful if you are directly working together with other
developers pushing feature branches on a shared remote. You typically
push feature branches to the remote so others can review. Once they are
satisfied and the branch is merged into the main branch it needs to be
deleted on the server.

Since we did not yet have a preselected default branch in the remote
delete dialog lets use the last merged branch if it is found on the
server.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>