dired-copy-filename-as-kill: Support project-relative names

* lisp/dired.el (dired-copy-filename-as-kill): Support new value
of ARG, to mean file name relative to project root (bug#76792).

* doc/emacs/dired.texi (Misc Dired Features): Mention it.

* etc/NEWS: Same.

doc/emacs/dired.texi

@@ -1868,8 +1868,9 @@ names of the marked (or next @var{n}) files into the kill ring, as if
 you had killed them with @kbd{C-w}.  The names are separated by a
 space.
 
-  With a zero prefix argument, this uses the absolute file name of
-each marked file.  With just @kbd{C-u} as the prefix argument, it uses
+  With a zero prefix argument, this uses the absolute file name of each
+marked file.  With prefix value 1, it uses names relative to the
+project root.  With just @kbd{C-u} as the prefix argument, it uses
 file names relative to the Dired buffer's default directory.  (This
 can still contain slashes if in a subdirectory.)  As a special case,
 if point is on a directory header line, @kbd{w} gives you the absolute

etc/NEWS

@@ -1211,6 +1211,10 @@ Without 'dired-hide-details-hide-absolute-location':
 When 'dired-make-directory-clickable' is non-nil, clicking on the base
 name of the directory now reverts the Dired buffer.
 
+*** 'dired-copy-filename-as-kill' supports project-relative names.
+With a new value of the prefix argument (1), this command copies file
+names relative to the root directory of the current project.
+
 ** Grep
 
 +++

lisp/dired.el

@@ -3478,6 +3478,7 @@ be quoted (with double quotes).  (When there's a single file, no
 quoting is done.)
 
 With a zero prefix arg, use the absolute file name of each marked file.
+With a prefix value 1, use the names relative to the current project root.
 With \\[universal-argument], use the file name relative to the Dired buffer's
 `default-directory'.  (This still may contain slashes if in a subdirectory.)
 
@@ -3489,8 +3490,13 @@ You can then feed the file name(s) to other commands with \\[yank]."
   (let* ((files
           (or (ensure-list (dired-get-subdir))
               (if arg
-                  (cond ((zerop (prefix-numeric-value arg))
+                  (cond ((eql 0 arg)
                          (dired-get-marked-files))
+                        ((eql 1 arg)
+                         (let ((root (project-root (project-current t))))
+                           (mapcar
+                            (lambda (file) (file-relative-name file root))
+                            (dired-get-marked-files))))
                         ((consp arg)
                          (dired-get-marked-files t))
                         (t