rebase: fix handling of restrict_revision

rebase: fix handling of restrict_revision

restrict_revision in the original shell script was an excluded revision
range.  It is also treated that way by the am-backend.  In the
conversion from shell to C (see commit 6ab54d17be3f ("rebase -i:
implement the logic to initialize $revisions in C", 2018-08-28)), the
interactive-backend accidentally treated it as a positive revision
rather than a negated one.

This was missed as there were no tests in the testsuite that tested an
interactive rebase with fork-point behavior.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>