rebase-i-p: delay saving current-commit to REWRITTEN if squashing

rebase-i-p: delay saving current-commit to REWRITTEN if squashing

If the current-commit was dumped to REWRITTEN, but then we squash the next
commit in to it, we have invalidated the HEAD was just written to REWRITTEN.
Instead, append the squash hash to current-commit and save both of them the next
time around.

Signed-off-by: Stephen Haberman <stephen@exigencecorp.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>