Move more logic to vc-ignore from vc-default-ignore

* lisp/vc/vc-dir.el (vc-dir-ignore):
Pass relative file names to vc-ignore.

* lisp/vc/vc.el (vc-ignore): Move the responsibility of
constructing the ignore pattern (right now, most often a relative
file name) using a file name received from the user, here.
(vc-default-ignore): ...from here (bug#37189, see discussion).
Also clarify the docstring.

lisp/vc/vc-dir.el

@@ -879,7 +879,9 @@ If a prefix argument is given, ignore all marked files."
 	   (vc-ignore (vc-dir-fileinfo->name filearg))
 	   t))
        vc-ewoc)
-    (vc-ignore (vc-dir-current-file))))
+    (vc-ignore
+     (file-relative-name (vc-dir-current-file))
+     default-directory)))
 
 (defun vc-dir-current-file ()
   (let ((node (ewoc-locate vc-ewoc)))

lisp/vc/vc.el

@@ -480,8 +480,8 @@
 ;;
 ;; - ignore (file &optional directory)
 ;;
-;;   Ignore FILE under the VCS of DIRECTORY (default is `default-directory').
+;;   Ignore FILE under DIRECTORY (default is 'default-directory').
-;;   FILE is a file wildcard.
+;;   FILE is a file wildcard relative to DIRECTORY.
 ;;   When called interactively and with a prefix argument, remove FILE
 ;;   from ignored files.
 ;;   When called from Lisp code, if DIRECTORY is non-nil, the
@@ -1406,40 +1406,43 @@ When called interactively, prompt for a FILE to ignore, unless a
 prefix argument is given, in which case prompt for a file FILE to
 remove from the list of ignored files."
   (interactive
-   (list
+   (let* ((backend (vc-responsible-backend default-directory))
-    (if (not current-prefix-arg)
+          (rel-dir
-        (read-file-name "File to ignore: ")
+           (condition-case nil
-      (completing-read
+               (file-name-directory
-       "File to remove: "
+                (vc-call-backend backend 'find-ignore-file
-       (vc-call-backend
+                                 default-directory))
-        (or (vc-responsible-backend default-directory)
+             (vc-not-supported
-            (error "Unknown backend"))
+              default-directory)))
-        'ignore-completion-table default-directory)))
+          (file (read-file-name "File to ignore: ")))
-    nil current-prefix-arg))
+     (when (and (file-name-absolute-p file)
+                (file-in-directory-p file rel-dir))
+       (setq file (file-relative-name file rel-dir)))
+     (list file
+           rel-dir
+           current-prefix-arg)))
   (let* ((directory (or directory default-directory))
 	 (backend (or (vc-responsible-backend default-directory)
                       (error "Unknown backend"))))
     (vc-call-backend backend 'ignore file directory remove)))
 
 (defun vc-default-ignore (backend file &optional directory remove)
-  "Ignore FILE under the VCS of DIRECTORY (default is `default-directory').
+  "Ignore FILE under DIRECTORY (default is `default-directory').
-FILE is a wildcard specification, either relative to
+FILE is a wildcard specification relative to DIRECTORY.
-DIRECTORY or absolute.
+
 When called from Lisp code, if DIRECTORY is non-nil, the
-repository to use will be deduced by DIRECTORY; if REMOVE is
+repository to use will be deduced by DIRECTORY.
-non-nil, remove FILE from ignored files.
+
-Argument BACKEND is the backend you are using."
+If REMOVE is non-nil, remove FILE from ignored files instead.
+
+Argument BACKEND is the backend to use."
   (let ((ignore
-	 (vc-call-backend backend 'find-ignore-file (or directory default-directory)))
+         (vc-call-backend backend
-	file-path root-dir pattern)
+                          'find-ignore-file
-    (setq file-path (expand-file-name file directory))
+                          (or directory default-directory))))
-    (setq root-dir (file-name-directory ignore))
-    (when (not (string= (substring file-path 0 (length root-dir)) root-dir))
-      (error "Ignore spec %s is not below project root %s" file-path root-dir))
-    (setq pattern (substring file-path (length root-dir)))
     (if remove
-	(vc--remove-regexp (concat "^" (regexp-quote pattern ) "\\(\n\\|$\\)") ignore)
+        (vc--remove-regexp (concat "^" (regexp-quote file) "\\(\n\\|$\\)") ignore)
-      (vc--add-line pattern ignore))))
+      (vc--add-line file ignore))))
 
 (defun vc-default-ignore-completion-table (backend file)
   "Return the list of ignored files under BACKEND."