mv: fix error for moving directory to another

mv: fix error for moving directory to another

If both directories D1 and D2 already exists, and further there is a
filesystem entity D2/D1, "git mv D1 D2" would fail, and we get an
error message that says:

    "cannot move directory over file, source=D1, destination=D2/D1"

regardless of the type of existing "D2/D1".  If it is a file, the
message is correct, but if it is a directory, it is not (we could
make the D2/D1 directory a union of its original contents and what
was in D1/, but that is not what we do).

The code that decies to issue the error message only checks for
existence of "D2/D1" and does not care what kind of thing sits at
the path.

Rephrase the message to say

    "destination already exists, source=D1, destination=D2/D1"

that would be suitable for any kind of thing being in the way.

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