refactor!(vc-gutter): move style defaults behind +pretty

BREAKING CHANGE: This changes moves its default configuration of the
fringe behind a +pretty flag. This won't cause breakage, but will cause
a (possibly unwanted) visual change: git-gutter will use its default
indicators (e.g. + and - symbols) instead of the continuous bars from
before.

To get back the old look, enable `+pretty` on the `:ui vc-gutter`
module.

This was done to make the defaults toggleable, to standardize +pretty,
to document issues with this customization that have been brought up by
members of the Emacs community, and to better document it in comments
for posterity.

modules/ui/vc-gutter/README.org

@@ -13,7 +13,13 @@ Supports Git, Svn, Hg, and Bzr.
 [[doom-contrib-maintainer:][Become a maintainer?]]
 
 ** Module flags
-/This module has no flags./
+- +pretty ::
+  Apply some stylistic defaults to the fringe, enabling thin bars in the fringe.
+  This look takes after the modern look of git-gutter in VSCode and Sublime
+  Text, without sacrificing on fringe width (which squeeze other indicators,
+  like flycheck's, flymake's, or flyspell's). However, this will look bad with
+  themes that invert the foreground/background of either git-gutter's or
+  diff-hl's faces (like modus-themes does).
 
 ** Packages
 - [[doom-package:][git-gutter-fringe]]

modules/ui/vc-gutter/config.el

@@ -16,6 +16,39 @@ This means subtle thin bitmaps on the left, an arrow bitmap for flycheck, and
 flycheck indicators moved to the right fringe.")
 
 
+;;
+;;; Default styles
+
+(when (featurep! +pretty)
+  ;; UI: make the fringe small enough that the diff bars aren't too domineering,
+  ;; while leaving enough room for other indicators.
+  (if (fboundp 'fringe-mode) (fringe-mode '8))
+  ;; UI: the gutter looks less cramped with some space between it and  buffer.
+  (setq-default fringes-outside-margins t)
+
+  ;; STYLE: Redefine fringe bitmaps to take up only half the horizontal space in
+  ;; the fringe. This way we avoid overbearingly large diff bars without having
+  ;; to shrink the fringe and sacrifice precious space for other fringe
+  ;; indicators (like flycheck or flyspell).
+  ;; TODO Extract these into a package with faces that themes can target.
+  (after! git-gutter-fringe
+    (define-fringe-bitmap 'git-gutter-fr:added [224]
+      nil nil '(center repeated))
+    (define-fringe-bitmap 'git-gutter-fr:modified [224]
+      nil nil '(center repeated))
+    (define-fringe-bitmap 'git-gutter-fr:deleted [128 192 224 240]
+      nil nil 'bottom))
+
+  ;; FIX: To minimize overlap between flycheck indicators and git-gutter/diff-hl
+  ;; indicators in the left fringe.
+  (after! flycheck
+    ;; let diff have left fringe, flycheck can have right fringe
+    (setq flycheck-indication-mode 'right-fringe)
+    ;; A non-descript, left-pointing arrow
+    (define-fringe-bitmap 'flycheck-fringe-bitmap-double-arrow
+      [16 48 112 240 112 48 16] nil nil 'center)))
+
+
 ;;
 ;;; Packages
 

templates/init.example.el

@@ -46,7 +46,7 @@
        ;;tabs              ; a tab bar for Emacs
        ;;treemacs          ; a project drawer, like neotree but cooler
        ;;unicode           ; extended unicode support for various languages
-       vc-gutter         ; vcs diff in the fringe
+       (vc-gutter +pretty) ; vcs diff in the fringe
        vi-tilde-fringe   ; fringe tildes to mark beyond EOB
        ;;window-select     ; visually switch windows
        workspaces        ; tab emulation, persistence & separate workspaces