gitk: Fix some corner cases in computing vrowmod and displayorder

gitk: Fix some corner cases in computing vrowmod and displayorder

First, insertfakerow and removefakerow weren't updating vrowmod,
and hence displayorder was not getting updated when it needed to,
in the case where the fake row was being inserted into or removed
from the last arc.  The comparison of varctok vs vtokmod was moved
into modify_arc for these cases (and for the call in rewrite_commit)
to avoid duplicating the extra code needed.  Second, the logic in
update_arcrows didn't end up truncating displayorder and unsetting
cached_commitrow if the first modified row was in the last arc.
This fixes these problems.

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