Install all packages in the git layer

The variable are now only used to enable or disable the
packages.
This approach allow better discovery of git layer packages in
helm-spacemacs

contrib/git/packages.el

@@ -15,32 +15,26 @@
     fringe-helper
     git-messenger
     git-timemachine
+    gist
+    github-browse-file
+    ;; not up to date
+    ;; helm-gist
     magit
+    magit-gh-pulls
     magit-gitflow
+    magit-svn
     smeargle
     )
   "List of all packages to install and/or initialize. Built-in packages
 which require an initialization must be listed explicitly in the list.")
 
-(when git-enable-github-support
-  (mapc (lambda (x) (push x git-packages))
-        '(
-          gist
-          github-browse-file
-          ;; not up to date
-          ;; helm-gist
-          magit-gh-pulls
-          )))
-
-(when git-enable-magit-svn-plugin
-    (push 'magit-svn git-packages))
-
 (if git-gutter-use-fringe
     (push 'git-gutter-fringe git-packages)
   (push 'git-gutter git-packages))
 
 (defun git/init-gist ()
   (use-package gist
+    :if git-enable-github-support
     :defer t
     :init
     (progn
@@ -248,12 +242,14 @@ which require an initialization must be listed explicitly in the list.")
 
 (defun git/init-magit-gh-pulls ()
   (use-package magit-gh-pulls ()
+    :if git-enable-github-support
     :defer t
     :init (add-hook 'magit-mode-hook 'turn-on-magit-gh-pulls)
     :config (spacemacs|diminish magit-gh-pulls-mode "Github-PR")))
 
 (defun git/init-github-browse-file ()
   (use-package github-browse-file
+    :if git-enable-github-support
     :defer t
     :init
     (evil-leader/set-key
@@ -267,6 +263,7 @@ which require an initialization must be listed explicitly in the list.")
 
 (defun git/init-magit-svn ()
   (use-package magit-svn
+    :if git-enable-magit-svn-plugin
     :commands turn-on-magit-svn
     :init (add-hook 'magit-mode-hook 'turn-on-magit-svn)
     :config