filter-branch: add a test for the commit removal example

filter-branch: add a test for the commit removal example

In the man page, there is an example which describes how to remove
single commits (although it keeps the changes which were not reverted
in the next non-removed commit). Better make sure that it works as
expected.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>