gitk: Recompute ancestor/descendent heads/tags when rereading refs

We weren't updating the desc_heads, desc_tags and anc_tags arrays when
rereading the set of heads/tags/etc.  The tricky thing to get right
here is restarting the computation correctly when we are only half-way
through it.

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