Pin magit to melpa-stable source

Latest master build of magit seems to have issues with the status
buffer, where it turns on syntax high-lighting for quoted strings and
other weirdness that leaks all over the status buffer.

modules/version-control/siren-forge.el

@@ -9,6 +9,7 @@
 (require 'siren-magit)
 
 (use-package forge
+  :pin melpa-stable
   :after magit)
 
 (provide 'siren-forge)

modules/version-control/siren-magit.el

@@ -10,6 +10,7 @@
 (require 'siren-flyspell)
 
 (use-package magit
+  :pin melpa-stable
   :bind
   ("C-x g". magit-status)