Add `project-ignores'

* lisp/progmodes/project.el (project-ignores): New generic
function, and an implementation for the	VC project type.

* lisp/progmodes/xref.el (xref--rgrep-command): Split, as a
variant of rgrep-default-command that handles a generic list of
ignores.
(xref-collect-matches): Use it, and pass through to it the value
of the newly added argument.
(xref-find-regexp): Handle ignored paths within the project.
Remove outdated comment.

* lisp/vc/vc.el (vc-default-ignore-completion-table):
Skip the comments and the empty lines.

1 files changed, 6 insertions, 2 deletions

diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 1bd04e13430..3b9e788b7ab 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1423,8 +1423,12 @@ Argument BACKEND is the backend you are using."
 
 (defun vc-default-ignore-completion-table (backend file)
   "Return the list of ignored files under BACKEND."
-  (vc--read-lines
-   (vc-call-backend backend 'find-ignore-file file)))
+  (cl-delete-if
+   (lambda (str)
+     ;; Commented or empty lines.
+     (string-match-p "\\`\\(?:#\\|[ \t\r\n]*\\'\\)" str))
+   (vc--read-lines
+    (vc-call-backend backend 'find-ignore-file file))))
 
 (defun vc--read-lines (file)
   "Return a list of lines of FILE."