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>

diff.c

@@ -1311,6 +1311,8 @@ static const char *get_textconv(struct diff_filespec *one)
 {
 	if (!DIFF_FILE_VALID(one))
 		return NULL;
+	if (!S_ISREG(one->mode))
+		return NULL;
 	diff_filespec_load_driver(one);
 	return one->driver->textconv;
 }

t/t4030-diff-textconv.sh

@@ -104,7 +104,7 @@ index ad8b3d2..67be421
 \ No newline at end of file
 EOF
 # make a symlink the hard way that works on symlink-challenged file systems
-test_expect_failure 'textconv does not act on symlinks' '
+test_expect_success 'textconv does not act on symlinks' '
 	echo -n frotz > file &&
 	git add file &&
 	git ls-files -s | sed -e s/100644/120000/ |