gitk: Cope better with getting commits that we have already seen

gitk: Cope better with getting commits that we have already seen

This fixes a bug in updating the graph after we have cherry-picked
a commit in gitk and then added some new stuff externally.  First,
we weren't updating viewincl with the new head added by the cherry-
pick.  Secondly, getcommitlines was doing bad things if it saw a
commit that was already in the graph (was already in an arc).  This
fixes both things.  If getcommitlines sees a commit that is already
in the graph, it ignores it unless it was not listed before and is
listed now.  In that case it doesn't assign it a new arc now, and
doesn't re-add the commit to its arc.

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