magit--git-wash: Support commands whose non-zero status isn't an error

This fixes a regression introduced by [1: 78a979f], which did not
take into account that "git diff --no-index" exits with 1 if there
are differences (like "diff" does).

Closes #5094.

1: 2023-03-18 78a979f
   Support displaying errors that occur when washing a section

lisp/magit-diff.el

@@ -2170,7 +2170,9 @@ keymap is the parent of their keymaps."
       (setq magit-git-global-arguments
             (append magit-diff--reset-non-color-moved
                     magit-git-global-arguments)))
-    (magit--git-wash #'magit-diff-wash-diffs keep-error cmd args)))
+    (magit--git-wash #'magit-diff-wash-diffs
+        (if (member "--no-index" args) 'wash-anyway keep-error)
+      cmd args)))
 
 (defun magit-diff--maybe-add-stat-arguments (args)
   (if (member "--stat" args)

lisp/magit-git.el

@@ -597,7 +597,8 @@ call function WASHER with ARGS as its sole argument."
         (magit-cancel-section)
       (unless (bolp)
         (insert "\n"))
-      (when (equal exit 0)
+      (when (or (equal exit 0)
+                (eq keep-error 'wash-anyway))
         (save-restriction
           (narrow-to-region beg (point))
           (goto-char beg)