only textconv regular files

only textconv regular files

We treat symlinks as text containing the results of the
symlink, so it doesn't make much sense to text-convert them.

Similarly gitlink components just end up as the text
"Subproject commit $sha1", which we should leave intact.

Note that a typechange may be broken into two parts: the
removal of the old part and the addition of the new. In that
case, we _do_ show the textconv for any part which is the
addition or removal of a file we would ordinarily textconv,
since it is purely acting on the file contents.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>