rebase: fix format.useAutoBase breakage

rebase: fix format.useAutoBase breakage

With `format.useAutoBase = true`, running rebase resulted in an
error:

	fatal: failed to get upstream, if you want to record base commit automatically,
	please use git branch --set-upstream-to to track a remote branch.
	Or you could specify base commit by --base=<base-commit-id> manually
	error:
	git encountered an error while preparing the patches to replay
	these revisions:

	    ede2467cdedc63784887b587a61c36b7850ebfac..d8f581194799ae29bf5fa72a98cbae98a1198b12

	As a result, git cannot rebase them.

Fix this by always passing `--no-base` to format-patch from rebase so
that the effect of `format.useAutoBase` is negated.

Reported-by: Christian Biesinger <cbiesinger@google.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>