mv: make non-directory destination error more clear

mv: make non-directory destination error more clear

If you try to "git mv" multiple files onto another
non-directory file, you confusingly get the "usage" message:

  $ touch one two three
  $ git add .
  $ git mv one two three
  usage: git mv [options] <source>... <destination>
  [...]

From the user's perspective, that makes no sense. They just
gave parameters that exactly match that usage!

This behavior dates back to the original C version of "git
mv", which had a usage message like:

  usage: git mv (<source> <destination> | <source>...  <destination>)

This was slightly less confusing, because it at least
mentions that there are two ways to invoke (but it still
isn't clear why what the user provided doesn't work).

Instead, let's show an error message like:

  $ git mv one two three
  fatal: destination 'three' is not a directory

We could leave the usage message in place, too, but it
doesn't actually help here. It contains no hints that there
are two forms, nor that multi-file form requires that the
endpoint be a directory. So it just becomes useless noise
that distracts from the real error.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>