sequencer: update reading author-script

sequencer: update reading author-script

Rather than abusing a strbuf to come up with an environment block,
let's just use the argv_array structure which serves the same
purpose much better.

While at it, rename the function to reflect the fact that it does
not really care exactly what environment variables are defined in
said file.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>