selectrum: improve file category commands...

now when pressing backspace on `/`, the input will go up one directory.

modules/completion/selectrum/TODO.org

@@ -17,9 +17,6 @@ gets us close but moves the cursor to the new screen which is undesirable.
 * PROJ List of things needed for Ivy parity
 ** TODO Icons
 https://github.com/minad/marginalia/issues/59
-** TODO make backspace work in filepath searching
-In ivy, backspace on =/foo/bar/!= (where =!= is the cursor) leads to =/foo/!=
-In selectrum, it leads to =/foo/bar!=
 ** TODO =SPC s B=
 ** TODO modules to look over
 - lookup

modules/completion/selectrum/config.el

@@ -14,9 +14,24 @@
   (defadvice! +selectrum-refresh-on-cycle (&rest _)
     :after 'marginalia-cycle
     (when (bound-and-true-p selectrum-mode) (selectrum-exhibit)))
+
+  (defun +selectrum/backward-updir ()
+    "Delete char before or go up directory for file cagetory selectrum buffers."
+    (interactive)
+    (if (and (eq (char-before) ?/)
+             (eq (selectrum--get-meta 'category) 'file))
+        (let ((new-path (minibuffer-contents)))
+          (delete-region (minibuffer-prompt-end) (point-max))
+          (insert (abbreviate-file-name
+                   (file-name-directory
+                    (directory-file-name
+                     (expand-file-name new-path))))))
+      (call-interactively 'backward-delete-char)))
+
   (map! :map selectrum-minibuffer-map
         "C-o"       #'embark-act
-        "C-c C-o" #'embark-export))
+        "C-c C-o"   #'embark-export
+        [backspace] #'+selectrum/backward-updir))
 
 (use-package! selectrum-prescient
   :when (featurep! +prescient)