bloom: parse commit before computing filters

bloom: parse commit before computing filters

When computing changed-path Bloom filters for a commit, we need to
know if the commit has a parent or not. If the commit is not parsed,
then its parent pointer will be NULL.

As far as I can tell, the only opportunity to reach this code
without parsing the commit is inside "test-tool bloom
get_filter_for_commit" but it is best to be safe.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>