Speed up project-files for Git projects

Speed up project-files for Git projects

* lisp/progmodes/project.el (project-files): New method.
Implementation for VC projects that uses 'git ls-files' or 'hg
status --all' for listing.  With gratitude to Tassilo Horn who has
done most of the legwork and wrote the first version of the code
(https://lists.gnu.org/archive/html/emacs-devel/2019-10/msg00069.html).
(project--vc-list-files): New function, to be used by the above.
(project--find-regexp-in-files):
Silence warnings about nonexistent files.