[version-control] Use :toggle to avoid installation of unused packages

Usage of the `:if` keyword with `use-package` does not prevent Spacemacs form
installing the package, it only prevents the loading in run-time. This commit
employs the `:toggle` feature of the configuration layer system to prevent the
unneeded packages from being installed in the first place.

This further improves upon 31324f68bb

CHANGELOG.develop

@@ -3133,6 +3133,7 @@ Other:
   - =browse-at-remote= which replaces =github-browse-file=
   (thanks Eugene Yaremenko)
 - Avoid loading all the diff packages (thanks to Sylvain Benner)
+- Avoided installing unused diff packages (thanks to Andriy Kmit)
 - Fixed error on =diff-hl-margin-mode= function being nil during startup
   (thanks to Voleking)
 **** Vue

layers/+source-control/version-control/README.org

@@ -8,7 +8,7 @@
 - [[#install][Install]]
   - [[#layer][Layer]]
 - [[#configuration][Configuration]]
-  - [[#differences-between-margin-programs][Differences between margin programs]]
+  - [[#differences-between-margin-packages][Differences between margin packages]]
 - [[#key-bindings][Key bindings]]
   - [[#vc-directory-buffer-commands][VC Directory buffer commands]]
   - [[#commit-message-editing-buffer][Commit message editing buffer]]
@@ -34,8 +34,12 @@ add =version-control= to the existing =dotspacemacs-configuration-layers= list i
 file.
 
 * Configuration
-You can choose the package to facilitate the diff transient-state and show margins
-by setting =version-control-diff-tool=
+You can choose the package to facilitate the diff transient-state and show
+margins by setting the =version-control-diff-tool= variable to one of the
+supported packages:
+- [[https://github.com/dgutov/diff-hl][diff-hl]]
+- [[https://github.com/syohex/emacs-git-gutter][git-gutter]]
+- [[https://github.com/nonsequitur/git-gutter-plus][git-gutter+]] (default)
 
 #+BEGIN_SRC emacs-lisp
   '(version-control :variables
@@ -57,9 +61,9 @@ To automatically enable diff margins in all buffers, set
                     version-control-global-margin t)
 #+END_SRC
 
-** Differences between margin programs
+** Differences between margin packages
 This layer contains generalized mappings for three diff margin packages:
-diff-hl, git-gutter, and git-gutter+.
+=diff-hl=, =git-gutter=, and =git-gutter+=.
 
 There are some differences between packages that might have some people prefer
 one over the other:

layers/+source-control/version-control/packages.el

@@ -12,15 +12,15 @@
 (setq version-control-packages
       '(
         browse-at-remote
-        (vc :location built-in)
+        (diff-hl            :toggle (eq 'diff-hl version-control-diff-tool))
         diff-mode
-        diff-hl
         evil-unimpaired
-        git-gutter
-        git-gutter+
-        git-gutter-fringe
-        git-gutter-fringe+
+        (git-gutter         :toggle (eq 'git-gutter version-control-diff-tool))
+        (git-gutter-fringe  :toggle (eq 'git-gutter version-control-diff-tool))
+        (git-gutter+        :toggle (eq 'git-gutter+ version-control-diff-tool))
+        (git-gutter-fringe+ :toggle (eq 'git-gutter+ version-control-diff-tool))
         (smerge-mode :location built-in)
+        (vc :location built-in)
         ))
 
 (defun version-control/init-vc ()
@@ -121,7 +121,6 @@
 
 (defun version-control/init-diff-hl ()
   (use-package diff-hl
-    :if (eq version-control-diff-tool 'diff-hl)
     :defer t
     :init
     (progn
@@ -143,7 +142,6 @@
 
 (defun version-control/init-git-gutter ()
   (use-package git-gutter
-    :if (eq version-control-diff-tool 'git-gutter)
     :defer t
     :init
     (progn
@@ -165,7 +163,6 @@
 
 (defun version-control/init-git-gutter-fringe ()
   (use-package git-gutter-fringe
-    :if (eq version-control-diff-tool 'git-gutter)
     :defer t
     :init
     (progn
@@ -222,7 +219,6 @@
 
 (defun version-control/init-git-gutter-fringe+ ()
   (use-package git-gutter-fringe+
-    :if (eq version-control-diff-tool 'git-gutter+)
     :defer t
     :init
     (progn