When growing the table, take the opportunity to "compact" it by removing
entries with NULL decoration.

Users may have "removed" decorations by passing NULL to
insert_decoration. An object's table entry can't actually be removed
during normal operation, as it would break the linear hash collision
search. But we can remove NULL decoration entries when rebuilding the
table.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>