sequencer (rebase -i): write an author-script file

sequencer (rebase -i): write an author-script file

When the interactive rebase aborts, it writes out an author-script file
to record the author information for the current commit. As we are about
to teach the sequencer how to perform the actions behind an interactive
rebase, it needs to write those author-script files, too.

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