gitk: Fix potential bug with fake commit IDs in renumbervarc

gitk: Fix potential bug with fake commit IDs in renumbervarc

When a fake row is added, we add its (fake) ID to the children list
for its (fake) parent.  If renumbervarc were to then renumber the
parent it would incorrectly use the fake child.  This avoids the
problem by adding a last_real_child procedure which won't return
a fake ID, and using it in renumbervarc.  For symmetry this also adds
a first_real_child procedure and uses it in ordertoken.

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