blame,cat-file --textconv: Don't assume mode is ``S_IFREF | 0664''

We need to get the correct mode when blame reads the source from the
working tree, the index, or trees.  This allows us to omit running
textconv filters on symbolic links.

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>
Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>