blackbox: adds a 'blackbox' command for viewing recent logs
Adds a 'hg blackbox' command for viewing the latest entries in the blackbox log.
By default it shows the last 10 entries, but -l allows the user to specify.
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
adda
$ cd ..