gitk: Add menu items for comparing a commit with the marked commit

gitk: Add menu items for comparing a commit with the marked commit

Sometimes one wants to see the different between two commits that are
a long distance apart in the graph display.  This is difficult to do
with the "Diff this -> selected" and "Diff selected -> this" menu
items because the need to maintain the selection means that one can't
use the find facilities or the reference list window to navigate from
one to the other.

This provides an alternative using the mark.  Having found one commit,
one marks it with the "Mark this commit" menu item, then navigates to
the other commit and uses the new "Diff this -> marked commit" and/or
"Diff marked commit -> this" menu items.

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