gitk: Make it possible to lay out all the rows we have received so far

gitk: Make it possible to lay out all the rows we have received so far

This arranges things so that we can do the layout all the way up to
the last commit that we have received from git log.  If we get more
commits we re-lay and redisplay (if necessary) the visible rows.

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