capistrano: allow overriding repo_url and branch (#1889)

config/deploy.rb

@@ -1,8 +1,8 @@
 lock '3.7.2'
+set :repo_url, ENV.fetch('REPO', 'https://github.com/tootsuite/mastodon.git')
+set :branch, ENV.fetch('BRANCH', 'master')
 
 set :application, 'mastodon'
-set :repo_url, 'https://github.com/tootsuite/mastodon.git'
-set :branch, 'master'
 set :rbenv_type, :user
 set :rbenv_ruby, File.read('.ruby-version').strip
 set :migration_role, :app