github: Add forge

Note that Forge binds the ' key to the Forge dispatch popup, but evil-magit
binds the ' key to the submodule dispatch popup (see
emacs-evil/evil-magit#54), and evil-magit's key
binding takes precedence with the vim editing style.  Because Forge's key
binding does not always work, it is omitted from the README.

Note also that Magit binds the % key to its worktree dispatch popup, but
Spacemacs binds the % key to the magit-gitflow popup, and Spacemacs's key
binding takes precedence.  Because there is no available key binding for
the worktree dispatch popup, Forge's key binding under that dispatch popup
is omitted from the README.

* CHANGELOG.develop:
* layers/+source-control/github/README.org: Update.
* layers/+source-control/github/packages.el (github-packages): Add forge.
(github/init-forge): Load forge after magit. Configure forge to use
spacemacs-cache-directory.
(github/init-magithub): Disabling injecting issues and pull-requests
sections if forge is installed.

CHANGELOG.develop

@@ -82,7 +82,7 @@ This file containes the change log for the next major version of Spacemacs.
 - New key binding ~SPC f i~ for =insert-file=.
 *** Layer changes
 **** GitHub
-- Add new package =magithub=
+- Add new packages =forge= and =magithub=
 - Remove package =github-browse-file= which has been replaced by
   =browse-at-remote= in =version-control= layer
 **** Git

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

@@ -13,6 +13,7 @@
 - [[#key-bindings][Key bindings]]
   - [[#magit-gh-pulls][magit-gh-pulls]]
   - [[#magithub][magithub]]
+  - [[#forge][forge]]
   - [[#gistel][gist.el]]
   - [[#clone-repositories][Clone repositories]]
 
@@ -21,7 +22,8 @@ This layers adds support for [[http://github.com][GitHub]].
 
 ** Features:
 - [[https://github.com/sigma/magit-gh-pulls][magit-gh-pulls]]: handy =magit= add-on to manage GitHub pull requests.
-- [[https://github.com/vermiculus/magithub][magithub]]: magit integrated interface to [[https://hub.github.com/][hub]] command line
+- [[https://github.com/vermiculus/magithub][magithub]]: Magit-integrated interface to [[https://hub.github.com/][hub]] command line
+- [[https://github.com/magit/forge][forge]]: integration with various forges, such as GitHub and GitLab.
 - [[https://github.com/defunkt/gist.el][gist.el]]: full-featured mode to browse and post GitHub gists.
 - [[https://github.com/sshaw/git-link][git-link]]: quickly generate URLs for commits or files.
 - [[https://github.com/dgtized/github-clone.el][github-clone]] allows for easy cloning and forking of repositories.
@@ -102,6 +104,16 @@ possible.
 | ~@ p~       | submit pull request upstream                     |
 | ~@ i~       | create an issue                                  |
 
+** forge
+In a =magit-status= buffer (~SPC g s~):
+
+| Key binding | Description                                                            |
+|-------------+------------------------------------------------------------------------|
+| ~F y~       | pull pull-requests and issues for the current repository               |
+| ~F Y~       | pull all notifications for the current repository's forge              |
+| ~b Y~       | create branch from pull-request                                        |
+| ~b y~       | create and check out branch from pull-request                          |
+
 ** gist.el
 
 | Key binding | Description                                   |

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

@@ -11,6 +11,7 @@
 
 (setq github-packages
       '(
+        forge
         gist
         github-clone
         github-search
@@ -21,6 +22,12 @@
         (spacemacs-github :location built-in)
         ))
 
+(defun github/init-forge ()
+  (use-package forge
+    :after magit
+    :init (setq forge-database-file (concat spacemacs-cache-directory
+                                            "forge-database.sqlite"))))
+
 (defun github/init-gist ()
   (use-package gist
     :defer t
@@ -94,7 +101,13 @@
         (magit-set "false" "--global" "magithub.status.includeStatusHeader")
         (magit-set "false" "--global" "magithub.status.includePullRequestsSection")
         (magit-set "false" "--global" "magithub.status.includeIssuesSection"))
-      (magithub-feature-autoinject t)
+      (magithub-feature-autoinject `(,@(when (not (package-installed-p 'forge))
+                                         '(issues-section
+                                           pull-requests-section))
+                                     completion
+                                     status-checks-header
+                                     commit-browse
+                                     pull-request-merge))
       (define-key magit-status-mode-map "@" #'magithub-dispatch-popup))))
 
 (defun github/init-spacemacs-github ()