git-rebase -i: clean-up error check codepath.

git-rebase -i: clean-up error check codepath.

After replaying a single change, the code performed a number of checks,
but some of them were for sanity checking, failures from which should
make the command abort, and others were checks to see if it should make
a new commit object.  Stringing them together with "&&" was wrong.

Signed-off-by: Junio C Hamano <gitster@pobox.com>