Bump default Git version to v2.28.0
This MR bumps the default Git version used by our Makefile and Dockerfile to use Git v2.28.0. The original bug with the reference-transaction hook which caused us to not be able to upgrade has been worked around. So we can at least start using that version, even if we don't yet make it mandatory.
Part of #2959 (closed)
Edited by Patrick Steinhardt