add parse_pathspec() that converts cmdline args to struct pathspec

add parse_pathspec() that converts cmdline args to struct pathspec

Currently to fill a struct pathspec, we do:

   const char **paths;
   paths = get_pathspec(prefix, argv);
   ...
   init_pathspec(&pathspec, paths);

"paths" can only carry bare strings, which loses information from
command line arguments such as pathspec magic or the prefix part's
length for each argument.

parse_pathspec() is introduced to combine the two calls into one. The
plan is gradually replace all get_pathspec() and init_pathspec() with
parse_pathspec(). get_pathspec() now becomes a thin wrapper of
parse_pathspec().

parse_pathspec() allows the caller to reject the pathspec magics that
it does not support. When a new pathspec magic is introduced, we can
enable it per command after making sure that all underlying code has no
problem with the new magic.

"flags" parameter is currently unused. But it would allow callers to
pass certain instructions to parse_pathspec, for example forcing
literal pathspec when no magic is used.

With the introduction of parse_pathspec, there are now two functions
that can initialize struct pathspec: init_pathspec and
parse_pathspec. Any semantic changes in struct pathspec must be
reflected in both functions. init_pathspec() will be phased out in
favor of parse_pathspec().

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