filter-branch: Add tests for submodules in tree-filter

filter-branch: Add tests for submodules in tree-filter

Add tests to make sure that:

1) a submodule can be removed and its content replaced with regular files
   ('rewrite submodule with another content'). This test passes only with
   the previous patch applied.

2) it is possible to replace submodule revision by direct index
   manipulation ('replace submodule revision'). Although it would be
   better to run such a filter in --index-filter, this test shows that
   this functionality is not broken by the previous patch. This succeeds
   both with and without the previous patch.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>