project-find-regexp using ripgrep

[Dmitry Gutov <dgutov@yandex.ru>]

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Here's a small patch I've been toying with, inspired by bug#41766.

In my testing, it makes the project search an order of magnitude faster. 
Probably due to smart parallelization.

If people confirm this experience, I'm going to install it (or something 
similar), even though, well, it would be nice to consolidate this search 
tool into something smarter, and done in one package only. But that for 
the future.

How to try:

- M-x project-find-regexp in your favorite project.
- If you're feeling scientific, evaluate something like

    (benchmark 1 '(project-find-regexp "grep-regexp-alist"))

- Change the argument to something else if you're searching something 
other than the Emacs project.
- Try it a couple of times.
- Note the reported timings.

- Install ripgrep (e.g. with 'apt install ripgrep').
- Apply the patch.
- [Rebuild], restart Emacs.
- Repeat the first several steps.

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diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 5b5fb4bc47..19fc362ddb 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -1246,12 +1246,20 @@ xref-matches-in-directory
 (declare-function tramp-tramp-file-p "tramp")
 (declare-function tramp-file-local-name "tramp")
 
+;; '-s' because 'git ls-files' can output broken symlinks.
+(defvar xref-grep-template
+  "xargs -0 rg <C> -nH --no-messages -g '!*/' -e <R>"
+  ;"xargs -0 grep <C> -snHE -e <R>"
+  )
+
 ;;;###autoload
 (defun xref-matches-in-files (regexp files)
   "Find all matches for REGEXP in FILES.
 Return a list of xref values.
 FILES must be a list of absolute file names."
   (cl-assert (consp files))
+  (require 'grep)
+  (defvar grep-highlight-matches)
   (pcase-let*
       ((output (get-buffer-create " *project grep output*"))
        (`(,grep-re ,file-group ,line-group . ,_) (car grep-regexp-alist))
@@ -1261,13 +1269,12 @@ xref-matches-in-files
        ;; first file is remote, they all are, and on the same host.
        (dir (file-name-directory (car files)))
        (remote-id (file-remote-p dir))
-       ;; 'git ls-files' can output broken symlinks.
-       (command (format "xargs -0 grep %s -snHE -e %s"
-                        (if (and case-fold-search
-                                 (isearch-no-upper-case-p regexp t))
-                            "-i"
-                          "")
-                        (shell-quote-argument (xref--regexp-to-extended regexp)))))
+       ;; The 'auto' default would be fine too, but ripgrep can't handle
+       ;; the options we pass in that case.
+       (grep-highlight-matches)
+       (command (grep-expand-template xref-grep-template
+                                      (xref--regexp-to-extended regexp)
+                                      regexp)))
     (when remote-id
       (require 'tramp)
       (setq files (mapcar