git-gui: Improve the branch delete confirmation dialogs.

git-gui: Improve the branch delete confirmation dialogs.

If the user is deleting a branch which is fully merged into the
selected test branch we should not confirm the delete with them,
the fact that the branch is fully merged means we can recover the
branch and no work will be lost.

If a branch is not fully merged, we should warn the user about which
branch(es) that is and continue deleting those which are fully merged.

We should only delete a branch if the user disables the merge check,
and in that case we should confirm with the user that a delete should
occur as this may cause them to lose changes.

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