Revert "doc/Makefile: drop doc-diff worktree and temporary files on "make clean""

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. Reported by Jonathan Nieder <jrnieder@gmail.com>.