t4011: remove SYMLINKS prerequisite
The part of the test that is about symbolic links in the index does not
require that the corresponding file system entry is actually a symbolic
link. Use test_ln_s_add to insert a symbolic link in the index. When
the file system does not support symbolic links, we actually have a
regular file in the worktree, which we can update as if it were a
symbolic link. diff-index picks up the symbolic link property from the
index.
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>