Run gitaly with 'serve' subcommand

Gitaly has added several subcommands over the last few years.
Execute Gitaly using the `serve` subcommand, rather then without a
subcommand, which implicitly calls `serve`.

Changelog: changed

Author: Will Chandler

files/gitlab-cookbooks/gitaly/templates/default/sv-gitaly-run.erb

@@ -25,7 +25,7 @@ exec chpst -e <%= @options[:env_dir] %> -P \
   -U <%= @options[:user] %>:<%= @options[:groupname] %> \
   -u <%= @options[:user] %>:<%= @options[:groupname] %> \
 <% if @options[:use_wrapper] %>
-  <%= @options[:wrapper_path] %> <%= @options[:bin_path] %> <%= @options[:config_path] %>
+  <%= @options[:wrapper_path] %> <%= @options[:bin_path] %> serve <%= @options[:config_path] %>
 <% else %>
-  <%= @options[:bin_path] %> <%= @options[:config_path] %>
+  <%= @options[:bin_path] %> serve <%= @options[:config_path] %>
 <% end %>