rebase -i: handle --continue more like non-interactive rebase

rebase -i: handle --continue more like non-interactive rebase

Non-interactive rebase requires the working tree to be clean, but
applies what is in the index without requiring the user to do it
herself.  Imitate that, but (since we are interactive, after all)
fire up an editor with the commit message.

It also fixes a subtle bug: a forgotten "continue" was removed, which
led to an infinite loop when continuing without remaining patches.

Both issues noticed by Frank Lichtenheld.

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