New project-save-some-buffers command

* lisp/progmodes/project.el (project-save-some-buffers): New command.
(project-prefix-map): Bind it to C-x p C-x s.
* etc/NEWS: Announce the new command and binding.

etc/NEWS

@@ -362,6 +362,10 @@ invoked standalone or from the 'project-switch-commands' dispatch menu.
 This user option describes projects that should always be skipped by
 'project-remember-project'.
 
+---
+*** New command 'project-save-some-buffers' bound to 'C-x p C-x s'.
+This is like 'C-x s', but only for this project's buffers.
+
 ** Registers
 
 *** New functions 'buffer-to-register' and 'file-to-register'.

lisp/progmodes/project.el

@@ -903,6 +903,7 @@ DIRS must contain directory names."
     (define-key map "x" 'project-execute-extended-command)
     (define-key map "o" 'project-any-command)
     (define-key map "\C-b" 'project-list-buffers)
+    (define-key map "\C-xs" 'project-save-some-buffers)
     map)
   "Keymap for project commands.")
 
@@ -1828,6 +1829,12 @@ Also see the `project-kill-buffers-display-buffer-list' variable."
           ((funcall query-user)
            (mapc #'kill-buffer bufs)))))
 
+;;;###autoload
+(defun project-save-some-buffers (arg)
+  "Like `save-some-buffers', but only for this project's buffers."
+  (interactive "P")
+  (save-some-buffers arg (save-some-buffers-root)))
+
 
 ;;; Project list