Fix git-mv for moving directories with 1 file

Fix git-mv for moving directories with 1 file

This is fixed by putting the file into @changedfiles/@addedfiles,
and not the directory this file is in.

Additionally, this fixes the behavior for attempting to overwrite
a file with a directory, and gives a message for all cases where
overwriting is not possible (file->dir,dir->file,dir->dir).

Thanks for Alexander Litvinov for noting this problem.

Signed-off-by: Josef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>