difftool: handle modified symlinks in dir-diff mode

difftool: handle modified symlinks in dir-diff mode

Detect the null object ID for symlinks in dir-diff so that difftool can
detect when symlinks are modified in the worktree.

Previously, a null symlink object ID would crash difftool.
Handle null object IDs as unknown content that must be read from
the worktree.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>