Add read_author_script() to sequencer.c based on the implementation in
builtin/am.c and update read_am_author_script() to use
read_author_script(). The sequencer code that reads the author script
will be updated in the next commit.
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>