Merge branch 'register-gitaly-flags' into 'master'

Register "gitaly_delta_islands" feature flag

See merge request gitlab-org/gitlab-ce!28181

lib/gitlab/gitaly_client.rb

@@ -32,7 +32,8 @@ module Gitlab
     CLIENT_NAME = (Sidekiq.server? ? 'gitlab-sidekiq' : 'gitlab-web').freeze
 
     SERVER_FEATURE_CATFILE_CACHE = 'catfile-cache'.freeze
-    SERVER_FEATURE_FLAGS = [SERVER_FEATURE_CATFILE_CACHE].freeze
+    # Server feature flags should use '_' to separate words.
+    SERVER_FEATURE_FLAGS = [SERVER_FEATURE_CATFILE_CACHE, 'delta_islands'].freeze
 
     MUTEX = Mutex.new