ibuffer: new filter to list buffers running a process

* lisp/ibuf-ext.el (ibuffer-filter-by-process): Add new filter (Bug#28825).
* lisp/ibuffer.el (ibuffer-mode-map): Bound it to '/E'.
; * etc/NEWS (Changes in Specialized Modes and Packages in Emacs 27.1):
; Announce this change.

etc/NEWS

@@ -56,6 +56,11 @@ whether '"' is also replaced in 'electric-quote-mode'.  If non-nil,
 
 * Changes in Specialized Modes and Packages in Emacs 27.1
 
+** Ibuffer
+
+---
+*** New filter ibuffer-filter-by-process; bound to '/E'.
+
 ** Edebug
 
 +++

lisp/ibuf-ext.el

@@ -1286,6 +1286,12 @@ currently used by buffers."
    :reader (read-from-minibuffer "Filter by name (regexp): "))
   (string-match qualifier (buffer-name buf)))
 
+;;;###autoload (autoload 'ibuffer-filter-by-process "ibuf-ext")
+(define-ibuffer-filter process
+    "Limit current view to buffers running a process."
+  (:description "process")
+  (get-buffer-process buf))
+
 ;;;###autoload (autoload 'ibuffer-filter-by-starred-name "ibuf-ext")
 (define-ibuffer-filter starred-name
     "Limit current view to buffers with name beginning and ending

lisp/ibuffer.el

@@ -522,6 +522,7 @@ directory, like `default-directory'."
     (define-key map (kbd "/ m") 'ibuffer-filter-by-used-mode)
     (define-key map (kbd "/ M") 'ibuffer-filter-by-derived-mode)
     (define-key map (kbd "/ n") 'ibuffer-filter-by-name)
+    (define-key map (kbd "/ E") 'ibuffer-filter-by-process)
     (define-key map (kbd "/ *") 'ibuffer-filter-by-starred-name)
     (define-key map (kbd "/ f") 'ibuffer-filter-by-filename)
     (define-key map (kbd "/ b") 'ibuffer-filter-by-basename)