gitk: Add a goto next/previous highlighted commit function

This is invoked by shift-down/shift-up.  It relies on a patch to
git-diff-tree that has recently gone into the git repository, commit
ID e0c97ca6 (without this it may just sit there doing waiting for
git-diff-tree when looking for the next/previous highlight).

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