gitk: Fix bug where arcs could get lost

gitk: Fix bug where arcs could get lost

Because we weren't fixing up vlastins when moving an arc from one
place to another, it was possible for us later to decide to move
an arc to the wrong place, and end up with an arc disconnected from
the rest of the graph.  This fixes it by updating vlastins when
necessary.

Signed-off-by: Paul Mackerras <paulus@samba.org>