ls: read metadata only if needed

ls: read metadata only if needed

If we just want to get a list of files, there's no point in
reading metadata, since it simply isn't needed. Pass want_meta
to the appropriate functions only when we need the metadata,
and similarly call vfs.augment_item_meta() only then.

Note that the previous vfs change really made this effective,
we'd otherwise lose the information.

Note also that unfortunately this is necessary even for the
--file-type command line option because otherwise we cannot
identify FIFOs; for everything else the git mode appears to
be sufficient.

Together, this reduces the number of blob reads from 62 to 8
for 'bup ls' on a trivial folder (the bup git's root folder).

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>