Code review comment for lp:~wgrant/launchpad/bug-598345-restrict-dep-contexts

Please say /why/ here:
+ # We want the external dependency lines to show up second: after the
174 + # archive itself, but before any other dependencies.
175 + return [sources_list_lines[0]] + external_dep_lines + \

e.g. 'for an unknown reason We want ...