Fix doomemacs#5060: don't hash relative backup file paths

Also refactors undo-fu-session's make-hashed-file-path advise to use
make-backup-file-name-1.

core/core-editor.el

@@ -154,11 +154,22 @@ or file path may exist now."
 
 ;; HACK Does the same for Emacs backup files, but also packages that use
 ;;      `make-backup-file-name-1' directly (like undo-tree).
-(defadvice! doom-make-hashed-backup-file-name-a (args)
+(defadvice! doom-make-hashed-backup-file-name-a (orig-fn file)
   "A few places use the backup file name so paths don't get too long."
-  :filter-args #'make-backup-file-name-1
-  (setcar args (sha1 (car args)))
-  args)
+  :around #'make-backup-file-name-1
+  (let ((alist backup-directory-alist)
+        backup-directory)
+    (while alist
+      (let ((elt (pop alist)))
+        (if (string-match (car elt) file)
+            (setq backup-directory (cdr elt)
+                  alist nil))))
+    (let ((file (funcall orig-fn file)))
+      (if (or (null backup-directory)
+              (not (file-name-absolute-p backup-directory)))
+          file
+        (expand-file-name (sha1 (file-name-nondirectory file))
+                          (file-name-directory file))))))
 
 
 ;;

modules/emacs/undo/config.el

@@ -31,10 +31,16 @@
   :config
   (setq undo-fu-session-incompatible-files '("\\.gpg$" "/COMMIT_EDITMSG\\'" "/git-rebase-todo\\'"))
 
-  ;; HACK Fix #4993: prevent file names that are too long for the filesystem.
+  ;; HACK Fix #4993: we've advised `make-backup-file-name-1' to produced SHA1'ed
+  ;;      filenames to prevent file paths that are too long, so we force
+  ;;      `undo-fu-session--make-file-name' to use it instead of its own
+  ;;      home-grown overly-long-filename generator.
   ;; TODO PR this upstream; should be a universal issue
-  (advice-add #'undo-fu-session--make-file-name
-              :filter-args #'doom-make-hashed-backup-file-name-a)
+  (defadvice! +undo-fu-make-hashed-session-file-name-a (file)
+    :override #'undo-fu-session--make-file-name
+    (let ((backup-directory-alist `(("." . ,undo-fu-session-directory))))
+      (concat (make-backup-file-name-1 file)
+              (if undo-fu-session-compression ".gz" ".el"))))
 
   ;; HACK Use the faster zstd to compress undo files instead of gzip
   (when (executable-find "zstd")