blame: show "previous" information in --porcelain/--incremental format

When the final blame is laid for a line to a <commit, path> pair, it also
gives a "previous" information to --porcelain and --incremental output
format.  It gives the parent commit of the blamed commit, _and_ a path in
that parent commit that corresponds to the blamed path --- in short, it is
the origin that would have been blamed (or passed blame through) for the
line _if_ the blamed commit did not change that line.

This unfortunately makes sanity checking of refcount quite complex, so I
ripped it out for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>