Document changes and release 0.6.4
doc: Fix git-pbuilder example

Closes: #725875
Be more robust about git status output changes

by using --porcelain. Heavily based on a patch by rian m. carlson

Closes: #726260
Don't export DEB_VENDOR="Debian"

Tests now take Ubuntu versions into consideration.

Closes: #723756
Bump standards version
Document changes and release 0.6.5