gitk: Fix bug in assigning row numbers to arcs

gitk: Fix bug in assigning row numbers to arcs

We weren't setting vtokmod and varcmod in renumbervarc, so after a
call to renumbervarc we sometimes weren't reassigning row numbers to
all the arcs whose row numbers had changed.  This fixes it.

This also collapses layoutmore and showstuff into one procedure and
gets rid of the phase variable.

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