pathspec: simpler logic to prefix original pathspec elements

pathspec: simpler logic to prefix original pathspec elements

The logic used to prefix an original pathspec element with 'prefix'
magic is more general purpose and can be used for more than just short
magic.  Remove the extra code paths and rename 'prefix_short_magic' to
'prefix_magic' to better indicate that it can be used in more general
situations.

Also, slightly change the logic which decides when to prefix the
original element in order to prevent a pathspec of "." from getting
converted to "" (empty string).

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>