rebase -i: be careful to wrap up fixup/squash chains

rebase -i: be careful to wrap up fixup/squash chains

When an interactive rebase was stopped at the end of a fixup/squash
chain, the user might have edited the commit manually before continuing
(with either `git rebase --skip` or `git rebase --continue`, it does not
really matter which).

We need to be very careful to wrap up the fixup/squash chain also in
this scenario: otherwise the next fixup/squash chain would try to pick
up where the previous one was left.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>