gitk: Select something appropriate on cherry-pick, branch reset and checkout

gitk: Select something appropriate on cherry-pick, branch reset and checkout

This makes gitk select the new commit when cherry-picking, and select
the new checked-out head when resetting or checking out a branch.
This feels more natural because the user is usually more interested
in that commit now than whatever was selected before.

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