Add :emacs undo module

Moves undo-fu/undo-tree out of core and allows uses to choose one or the
other.

Relevant to #2339

modules/emacs/undo/README.org

@@ -0,0 +1,54 @@
+#+TITLE:   emacs/undo
+#+DATE:    April 13, 2020
+#+SINCE:   v3.0.0
+#+STARTUP: inlineimages nofold
+
+* Table of Contents :TOC_3:noexport:
+- [[#description][Description]]
+  - [[#maintainers][Maintainers]]
+  - [[#module-flags][Module Flags]]
+  - [[#plugins][Plugins]]
+  - [[#hacks][Hacks]]
+- [[#prerequisites][Prerequisites]]
+- [[#features][Features]]
+- [[#configuration][Configuration]]
+- [[#troubleshooting][Troubleshooting]]
+
+* Description
+This module augments Emacs' built-in undo system to be more intuitive.
+
+** Maintainers
+This module has no dedicated maintainers.
+
+** Module Flags
++ =+tree= Uses ~undo-tree~ instead of ~undo-fu~, which is a little less stable,
+  but offers branching undo history and a visualizer for navigating it.
+
+** Plugins
++ [[https://gitlab.com/ideasman42/emacs-undo-fu][undo-fu]]
++ [[https://gitlab.com/ideasman42/emacs-undo-fu-session][undo-fu-session]]
++ [[https://github.com/emacsmirror/undo-tree][undo-tree]] (=+tree=)
+
+** Hacks
++ Both undo-fu and undo-tree have been modified to use zstd to compress undo
+  history if it is available.
++ undo-tree only
+  + Text properties are stripped from undo history to shrink it.
+  + Undo-tree is too chatty about saving its history files. This has be
+    "silenced". i.e. It's visible in \*Messages\*, but won't appear in your
+    minibuffer.
++ unfo-fu only
+  + Doom defines =undo-fu-mode= to make it easier to add hooks/mode-local
+    keybinds.
+
+* Prerequisites
+This module has no prereqisites.
+
+* TODO Features
+# An in-depth list of features, how to use them, and their dependencies.
+
+* TODO Configuration
+# How to configure this module, including common problems and how to address them.
+
+* TODO Troubleshooting
+# Common issues and their solution, or places to look for help.

modules/emacs/undo/config.el

@@ -0,0 +1,89 @@
+;;; emacs/undo/config.el -*- lexical-binding: t; -*-
+
+(use-package! undo-fu
+  :unless (featurep! +tree)
+  :after-call doom-switch-buffer after-find-file
+  :init
+  (after! undo-tree
+    (global-undo-tree-mode -1))
+  :config
+  ;; Store more undo history to prevent loss of data
+  (setq undo-limit 400000
+        undo-strong-limit 3000000
+        undo-outer-limit 3000000)
+
+  (define-minor-mode undo-fu-mode
+    "Enables `undo-fu' for the current session."
+    :keymap (let ((map (make-sparse-keymap)))
+              (define-key map [remap undo] #'undo-fu-only-undo)
+              (define-key map [remap redo] #'undo-fu-only-redo)
+              map)
+    :init-value t
+    :global t))
+
+
+(use-package! undo-fu-session
+  :unless (featurep! +tree)
+  :hook (undo-fu-mode . global-undo-fu-session-mode)
+  :preface
+  (setq undo-fu-session-directory (concat doom-cache-dir "undo-fu-session/")
+        undo-fu-session-incompatible-files '("/COMMIT_EDITMSG\\'" "/git-rebase-todo\\'"))
+
+  ;; HACK We avoid `:config' here because `use-package's `:after' complicates
+  ;;      the load order of a package's `:config' block and makes it impossible
+  ;;      for the user to override its settings with merely `after!' (or
+  ;;      `eval-after-load'). See jwiegley/use-package#829.
+  (after! undo-fu-session
+    ;; HACK Use the faster zstd to compress undo files instead of gzip
+    (when (executable-find "zstd")
+      (defadvice! doom--undo-fu-session-use-zstd-a (filename)
+        :filter-return #'undo-fu-session--make-file-name
+        (if undo-fu-session-compression
+            (concat (file-name-sans-extension filename) ".zst")
+          filename)))))
+
+
+(use-package! undo-tree
+  :when (featurep! +tree)
+  ;; Branching & persistent undo
+  :after-call doom-switch-buffer-hook after-find-file
+  :config
+  (setq undo-tree-visualizer-diff t
+        undo-tree-auto-save-history t
+        undo-tree-enable-undo-in-region t
+        ;; Increase undo-limits by a factor of ten to avoid emacs prematurely
+        ;; truncating the undo history and corrupting the tree. See
+        ;; https://github.com/syl20bnr/spacemacs/issues/12110
+        undo-limit 800000
+        undo-strong-limit 12000000
+        undo-outer-limit 120000000
+        undo-tree-history-directory-alist
+        `(("." . ,(concat doom-cache-dir "undo-tree-hist/"))))
+
+  ;; Compress undo-tree history files with zstd, if available. File size isn't
+  ;; the (only) concern here: the file IO barrier is slow for Emacs to cross;
+  ;; reading a tiny file and piping it in-memory through zstd is *slightly*
+  ;; faster than Emacs reading the entire undo-tree file from the get go (on
+  ;; SSDs). Whether or not that's true in practice, we still enjoy zstd's ~80%
+  ;; file savings (these files add up over time and zstd is so incredibly fast).
+  (when (executable-find "zstd")
+    (defadvice! doom--undo-tree-make-history-save-file-name-a (file)
+      :filter-return #'undo-tree-make-history-save-file-name
+      (concat file ".zst")))
+
+  ;; Strip text properties from undo-tree data to stave off bloat. File size
+  ;; isn't the concern here; undo cache files bloat easily, which can cause
+  ;; freezing, crashes, GC-induced stuttering or delays when opening files.
+  (defadvice! doom--undo-tree-strip-text-properties-a (&rest _)
+    :before #'undo-list-transfer-to-tree
+    (dolist (item buffer-undo-list)
+      (and (consp item)
+           (stringp (car item))
+           (setcar item (substring-no-properties (car item))))))
+
+  ;; Undo-tree is too chatty about saving its history files. This doesn't
+  ;; totally suppress it logging to *Messages*, it only stops it from appearing
+  ;; in the echo-area.
+  (advice-add #'undo-tree-save-history :around #'doom-shut-up-a)
+
+  (global-undo-tree-mode +1))

modules/emacs/undo/packages.el

@@ -0,0 +1,7 @@
+;; -*- no-byte-compile: t; -*-
+;;; emacs/undo/packages.el
+
+(if (not (featurep! +tree))
+    (package! undo-tree :pin "5b6df03781")
+  (package! undo-fu :pin "0c34b6747e")
+  (package! undo-fu-session :pin "b808ef0cdc"))