Revert part of 744dacd (builtin-mv: minimum fix to avoid losing files)

Revert part of 744dacd (builtin-mv: minimum fix to avoid losing files)

When get_pathspec() was originally made absolute-path capable,
we botched the interface to it, without dying inside the function
when given a path that is outside the work tree, and made it the
responsibility of callers to check the condition in a roundabout
way.  This is made unnecessary with the previous patch.

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