difftool: Use symlinks when diffing against the worktree

difftool: Use symlinks when diffing against the worktree

Teach difftool's --dir-diff mode to use symlinks to represent
files from the working copy, and make it the default behavior
for the non-Windows platforms.

Using symlinks is simpler and safer since we do not need to
worry about copying files back into the worktree.
The old behavior is still available as --no-symlinks.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>