* files.el (locate-dominating-stop-dir-regexp): New var.

* files.el (locate-dominating-stop-dir-regexp): New var.
(locate-dominating-file): Change arg from a regexp to a file name.
Rewrite using the vc-find-root code to avoid directory-files which is
too slow.  Obey locate-dominating-stop-dir-regexp.
Don't pay attention to changes in owner.
(project-find-settings-file): Adjust call to locate-dominating-file.

* progmodes/flymake.el (flymake-find-buildfile):
Adjust call to locate-dominating-file.

* vc-hooks.el (vc-find-root): Use locate-dominating-file.
(vc-ignore-dir-regexp): Use locate-dominating-stop-dir-regexp.