The old shell version used show-branch --independent to filter for the
ones that cannot be reached from any other reference.
The new C version uses reduce_heads() from commit.c for this, so
add test to ensure it works as expected.
Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
