parse_pathspec: add PATHSPEC_PREFER_{CWD,FULL} flags

parse_pathspec: add PATHSPEC_PREFER_{CWD,FULL} flags

We have two ways of dealing with empty pathspec:

1. limit it to current prefix
2. match the entire working directory

Some commands go with #1, some #2. get_pathspec() and parse_pathspec()
only support #1. Make parse_pathspec() reject empty pathspec by
default. #1 and #2 can be specified via new flags. This makes it more
expressive about default behavior at command level.

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