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

This option will make the generated commit IDs deterministic, greatly
reducing the number of repository objects created when calling `make
update` repeatedly.

Using a loop makes the script a bit nicer, and it works around a git bug
corrupting patch dates when multiple patches are applied at once.

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