sequencer: fix quoting in write_author_script

sequencer: fix quoting in write_author_script

Single quotes should be escaped as \' not \\'. The bad quoting breaks
the interactive version of 'rebase --root' (which is used when there
is no '--onto' even if the user does not specify --interactive) for
authors that contain "'" as sq_dequote() called by read_author_ident()
errors out on the bad quoting.

For other interactive rebases this only affects external scripts that
read the author script and users whose git is upgraded from the shell
version of rebase -i while rebase was stopped when the author contains
"'". This is because the parsing in read_env_script() expected the
broken quoting.

This patch includes code to handle the broken quoting when
git has been upgraded while rebase was stopped. It does this by
detecting the missing "'" at the end of the GIT_AUTHOR_DATE line to see
if it should dequote \\' as "'". Note this is only implemented for
normal picks, not for creating a new root commit (rebase will stop with
an error complaining out bad quoting in that case).

The fallback code has been manually tested by reverting both the quoting
fixes in write_author_script() and the previous fix for the missing "'"
at the end of the GIT_AUTHOR_DATE line and running
t3404-rebase-interactive.sh.

Ideally rebase and am would share the same code for reading and
writing the author script, but this commit just fixes the immediate
bug.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>