bug#69532: mv's new -x option should be made orthogonal to -t/-T/default

[Paul Eggert]

Although I like the idea of exposing file swaps to the user, the first 
cut of 'mv -x' has significant problems.

I expect 'mv -x A B' to act like 'mv A B' except the destination must 
exist and is renamed back to A. However, this is not true for 'mv -x A 
B' when B is a directory; it renames B to A rather than renaming B/A to 
A as I expect. That is, 'mv -x' acts as if -T (--no-target-directory) is 
also specified. There is no way to get mv's traditional behavior, or to 
get mv -t behavior.

To fix this, 'mv -x' should respect the usual mv behavior with respect 
to directories. For example, when D is a directory 'mv -x A B C D' 
should act like 'mv A B C D' except that D's old entries should be 
renamed back to A B and C. And the -t and -T options should work with -x 
the same way they work when -x is not specified.

This needs to happen before the next coreutils release, to avoid 
confusion about 'mv -x'.