Revert +hub flag, but disable magithub by default

Must be enabled on a per-project basis. You can change this behavior by
setting +magit-hub-enabled-by-default to non-nil (before magit is
loaded).

Magithub has been made opt-in because:

1. Magithub is imposing, asking the user for a token, especially for
   users who don't use github (much or at all), but may occasionally
   have a project with a github remote.
2. magithub is really slow on first load for medium-to-large repos.
3. It's really easy to enable it through the magithub popup (H C e).
   magithub.enabled is saved into the project's .git/config file, so the
   setting will persist.

Also added a docstring to +magit-hub-features

init.example.el

@@ -56,7 +56,7 @@
       ;gist              ; interacting with github gists
       ;macos             ; MacOS-specific commands
       ;make              ; run make tasks from Emacs
-      ;(magit +hub)      ;
+      ;magit             ;
       ;password-store    ; password manager for nerds
        pdf               ; pdf enhancements
       ;prodigy           ; FIXME managing external services & code builders

modules/tools/magit/autoload.el

@@ -51,3 +51,19 @@ format."
                                .owner.login .name)))
              (magithub-clone repo dir)))
     (magit-clone url-or-repo dir)))
+
+
+;;
+;; Advice
+;;
+
+;;;###autoload
+(defun +magit*hub-settings--format-magithub.enabled ()
+  "Change the setting to display 'false' as its default."
+  (magit--format-popup-variable:choices "magithub.enabled" '("true" "false") "false"))
+
+;;;###autoload
+(defun +magit*hub-enabled-p ()
+  "Disables magithub by default."
+  (magithub-settings--value-or "magithub.enabled" nil
+    #'magit-get-boolean))

modules/tools/magit/config.el

@@ -1,8 +1,12 @@
 ;;; tools/magit/config.el -*- lexical-binding: t; -*-
 
+(defvar +magit-hub-enable-by-default nil
+  "Whether or not to enable magithub features for all projects by default.")
+
 (defvar +magit-hub-features
   '(pull-request-merge commit-browse completion)
-  "TODO")
+  "What features to initialize when `magithub' is loaded. Set this to `t' to
+load everything.")
 
 
 ;;
@@ -29,7 +33,6 @@
 
 
 (def-package! magithub
-  :when (featurep! +hub)
   :after magit
   :preface
   (setq magithub-dir (concat doom-etc-dir "magithub/"))
@@ -37,6 +40,15 @@
   (setq magithub-clone-default-directory "~/"
         magithub-preferred-remote-method 'clone_url)
   :config
+  (unless +magit-hub-enable-by-default
+    ;; Disable magit by default. Can be enabled through magithub settings popup,
+    ;; or setting `+magit-hub-enable-by-default'.
+    (advice-add #'magithub-enabled-p :override #'+magit*hub-enabled-p)
+    ;; I don't use `magithub-settings--simple' to redefine this because it
+    ;; changes the order of settings. Obnoxious, but the alternative is even
+    ;; more so.
+    (advice-add #'magithub-settings--format-magithub.enabled
+                :override #'+magit*hub-settings--format-magithub.enabled))
   (magithub-feature-autoinject +magit-hub-features))