The default abbreviation length depends on factors like user configuration,
git version and number of repository objects. Use unabbreviates IDs to make
patch generation more reproducible.

git 2.9 enables rename detection by default, changing the content of some
of our patch files. Explicitly disable it.

Suggested-by: Steffen Möller <steffen_moeller@gmx.de>

This fixes `make update` not working correctly when GLUON_SITEDIR is set
manually.