builtin-mv: minimum fix to avoid losing files

builtin-mv: minimum fix to avoid losing files

An incorrect command "git mv subdir /outer/space" threw the
subdirectory to outside of the repository and then noticed that
/outer/space/subdir/ would be outside of the repository.  The
error checking is backwards.

This fixes the issue by being careful about use of the return
value of get_pathspec().  Since the implementation already has
handcrafted loop to munge each path on the command line, we use
prefix_path() instead.

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