filter-branch: Extend test to show rewriting bug

filter-branch: Extend test to show rewriting bug

This extends the --subdirectory-filter test in t7003 to demonstrate a
rewriting bug: when rewriting two refs A and B such that B is an
ancestor of A, it fails to rewrite B.

The underlying issue is that the rev-list invocation at
git-filter-branch.sh:332 more or less boils down to

  git rev-list B --boundary ^A

which outputs nothing because B is an ancestor of A.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>