rebase -i: commit when continuing after "edit"

rebase -i: commit when continuing after "edit"

When doing an "edit" on a commit, editing and git-adding some files,
"git rebase -i" complained about a missing "author-script".  The idea was
that the user would call "git commit --amend" herself.

But we can be nice and do that for the user.

Noticed by Dmitry Potapov.

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