* lisp/progmodes/project.el (project-compile): Ignore
compile-command from vc-compilation-mode (bug#79658).
* lisp/vc/vc-dispatcher.el (vc-compilation-mode-hook): New
variable.
(vc-compilation-mode): Promote to its own derived mode. Don't
use define-derived-mode in order to preserve the BACKEND
argument to this function. Don't give it its own major mode
map, et cetera, for now.