This reverts commit 6f924265a0bf6efa677e9a684cebdde958e5ba06, which
started to require that we have an executable git available in order
to say "make clean", which gives us a chicken-and-egg problem.
Having to have Git installed, or be in a repository, in order to be
able to run an optional "doc-diff" tool is fine. Requiring either
in order to run "make clean" is a different story.