Git: Only show Tags root node when tags are enabled and exist

Create it on demand. Remove on clear

Change-Id: Ic29e82a859f99b5d739c25be83aa6c40a1ee2cc8
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>