sequencer: use read_author_script()

sequencer: use read_author_script()

Use the new function added in the last commit to read the author
script, updating read_env_script() and read_author_ident(). We now
have a single code path that reads the author script for am and all
flavors of rebase. This changes the behavior of read_env_script() as
previously it would set any environment variables that were in the
author-script file. Now it is an error if the file contains other
variables or any of GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL and
GIT_AUTHOR_DATE are missing. This is what am and the non interactive
version of rebase have been doing for several years so hopefully it
will not cause a problem for interactive rebase users. The advantage
is that we are reusing existing code from am which uses sq_dequote()
to properly dequote variables. This fixes potential problems with user
edited scripts as read_env_script() which did not track quotes
properly.

This commit also removes the fallback code for checking for a broken
author script after git is upgraded when a rebase is stopped. Now that
the parsing uses sq_dequote() it will reliably return an error if the
quoting is broken and the user will have to abort the rebase and
restart. This isn't ideal but it's a corner case and the detection of
the broken quoting could be confused by user edited author scripts.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>