difftool: handle unmerged files in dir-diff mode

difftool: handle unmerged files in dir-diff mode

When files are unmerged they can show up as both unmerged and
modified in the output of `git diff --raw`.  This causes
difftool's dir-diff to create filesystem entries for the same
path twice, which fails when it encounters a duplicate path.

Ensure that each worktree path is only processed once.
Add a test to demonstrate the breakage.

Reported-by: Jan Smets <jan@smets.cx>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>