update: show message when a bookmark is activated by update
When updating to a bookmark, mention that the bookmark is now
active. This is a reminder that update does not move the
current bookmark if an explicit target is given - instead
it activates that target.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
revision 0
branch default
phase draft
adda
$ cd ..