[core] provide keybindings for using narrow with an indirect buffer

This is useful if you want multiple narrowing in the same base buffer.

Fixes #12550 with proposed bindings by @duianto.

CHANGELOG.develop

@@ -521,6 +521,9 @@ Other:
     terminal mode, but no longer in GUI mode. (emacs18)
 *** Core changes
 - Improvements:
+  - Provide keybindings for using narrow with an indirect buffer, for when you
+    want narrowing multiple times in the same base buffer (thanks to Keith
+    Pinson and duianto)
   - Make =describe-text-properties= available in the help -> describe menu via
     ~SPC h d t~ (thanks to Keith Pinson)
   - Bind ~SPC c n~ to ~:cn~ / ~next-error~ and ~SPC c N~ to ~:cN~ / ~previous-error~

layers/+spacemacs/spacemacs-defaults/funcs.el

@@ -1724,3 +1724,24 @@ Decision is based on `dotspacemacs-line-numbers'."
            (mapcar 'cdr
                    (sort (mapcar (lambda (x) (cons (random) (concat x "\n"))) lines)
                          (lambda (a b) (< (car a) (car b))))))))
+
+
+;; narrow region
+
+(defun spacemacs/narrow-to-defun-indirect-buffer ()
+  (interactive)
+  (call-interactively 'clone-indirect-buffer)
+  (call-interactively 'narrow-to-defun)
+  (message "Function narrowed to an indirect buffer"))
+
+(defun spacemacs/narrow-to-page-indirect-buffer ()
+  (interactive)
+  (call-interactively 'clone-indirect-buffer)
+  (call-interactively 'narrow-to-page)
+  (message "Page narrowed to an indirect buffer"))
+
+(defun spacemacs/narrow-to-region-indirect-buffer ()
+  (interactive)
+  (call-interactively 'clone-indirect-buffer)
+  (call-interactively 'narrow-to-region)
+  (message "Region narrowed to an indirect buffer"))

layers/+spacemacs/spacemacs-defaults/keybindings.el

@@ -454,6 +454,9 @@
   "nr" 'narrow-to-region
   "np" 'narrow-to-page
   "nf" 'narrow-to-defun
+  "nR" 'spacemacs/narrow-to-region-indirect-buffer
+  "nP" 'spacemacs/narrow-to-page-indirect-buffer
+  "nF" 'spacemacs/narrow-to-defun-indirect-buffer
   "nw" 'widen)
 ;; toggle ---------------------------------------------------------------------
 (spacemacs|add-toggle highlight-current-line-globally