mv: remove use of deprecated 'get_pathspec()'

mv: remove use of deprecated 'get_pathspec()'

Convert the 'internal_copy_pathspec()' function to 'prefix_path()'
instead of using the deprecated 'get_pathspec()' interface.  Also,
rename 'internal_copy_pathspec()' to 'internal_prefix_pathspec()' to be
more descriptive of what the funciton is actually doing.

In addition to this, fix a memory leak caused by only duplicating some
of the pathspec elements.  Instead always duplicate all of the the
pathspec elements as an intermediate step (with modificationed based on
the passed in flags).  This way the intermediate strings can then be
freed after getting the result from 'prefix_path()'.

Signed-off-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>