[git-buildpackage] Updates to Debian Developers Reference about git-buildpackage

[Otto Kekäläinen]

Hi,

I noticed that the DDR was almost void of information about
git-buildpackage, despite it being the most common tool maintainers
use on daily basis. I addressed it by adding some info, and just
wanted to share here that
https://salsa.debian.org/debian/developers-reference/-/commit/5cc63e9c0d0e9efd81b0d53b84adf62ea3044dc8
got included in the latest version of
https://www.debian.org/doc/manuals/developers-reference/