t7800: readlink may not be available

The readlink(1) command is not available on all platforms (notably
not on AIX and HP-UX) and can be replaced in this test with the
"workaround"

ls -ld <name> | sed -e 's/.* -> //'

This is no universal readlink replacement but works in the
controlled test environment well enough.

Signed-off-by: Armin Kunaschik <megabreit@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7800-difftool.sh

@@ -423,7 +423,7 @@ write_script .git/CHECK_SYMLINKS <<\EOF
 for f in file file2 sub/sub
 do
 	echo "$f"
-	readlink "$2/$f"
+	ls -ld "$2/$f" | sed -e 's/.* -> //'
 done >actual
 EOF