gitk: Fix various bugs in the view support

gitk: Fix various bugs in the view support

- don't re-read refs when switching views, it's too slow; just do
  it if the user did File->Update
- make the view menu use the uifont
- if we have a graph line selected, unselect it before changing the view
- if a row is selected and appears in the new view, but we have to
  read in the new view, select that row when we come across it
- if no row was previously selected, or if we don't find the previously
  selected row in the new view, select the first row

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