filter-branch: fix errors caused by refs that point at non-committish

filter-branch: fix errors caused by refs that point at non-committish

"git filter-branch -- --all" prints error messages when processing refs that
point at objects that are not committish. Such refs can be created by
"git replace" with trees or blobs. And also "git tag" with trees or blobs can
create such refs.

Filter these problematic refs out early, before they are seen by the logic to
see which refs have been modified and which have been left intact (which is
where the unwanted error messages come from), and warn that these refs are left
unwritten while doing so.

Signed-off-by: Yuki Kokubun <orga.chem.job@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>