Add script to update ChangeLog from git log.

This patch adds update-changelog.pl, a script which adds the subject line and
author of every commit since ChangeLog was last touched by git, in a style
consistent with the entries up to version 0.92.0.
Update ChangeLog, from version 0.92.0+ to most recent commit.

Generated by the new update-changelog.pl script.
wmaker: fix arbitrary shell command injection

Workspace background pref can be tricked to run arbitrary cmds.