Previously, old mirrors are only removed if the requested spec is
stable, and if the mirror lines only have two leading spaces. This leads
to stale mirror line(s) when the formula a stable block like
stable do
url "http://example.com/v1.0.tar.gz"
mirror "http://example.net/v1.0.tar.gz"
end
where the mirror line is lead by four spaces.
In this commit, we discard the /(^ mirror .*\n)?/ pattern, and instead
create a pattern with the exact url and flexible leading spaces for each
mirror of the requested spec.
3ebd7df62d