sequencer (rebase -i): allow continuing with staged changes

sequencer (rebase -i): allow continuing with staged changes

When an interactive rebase is interrupted, the user may stage changes
before continuing, and we need to commit those changes in that case.

Please note that the nested "if" added to the sequencer_continue() is
not combined into a single "if" because it will be extended with an
"else" clause in a later patch in this patch series.

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