When a commit changes a path P that used to be a file to a directory
and creates a new path P/X in it, "git show" would say that file P
was removed and file P/X was created for such a commit.

However, if we compare two directories, D1 and D2, where D1 has a
file D1/P in it and D2 has a directory D2/P under which there is a
file D2/P/X, and ask "git diff --no-index D1 D2" to show their
differences, we simply get a refusal "file/directory conflict".

Surely, that may be what GNU diff does, but we can do better and it
is easy to do so.

Signed-off-by: Junio C Hamano <gitster@pobox.com>