magic pathspec: add tentative ":/path/from/top/level" pathspec support

Support ":/" magic string that can be prefixed to a pathspec element to
say "this names the path from the top-level of the working tree", when
you are in the subdirectory.

For example, you should be able to say:

    $ edit Makefile ;# top-level
    $ cd Documentation
    $ edit git.txt ;# in the subdirectory

and then do one of three things, still inside the subdirectory:

    $ git add -u .  ;# add only Documentation/git.txt
    $ git add -u :/ ;# add everything, including paths outside Documentation
    $ git add -u    ;# whatever the default setting is.

To truly support magic pathspec, the API needs to be restructured so that
get_pathspec() and init_pathspec() are unified into one call.  Currently,
the former just prefixes the user supplied pathspec with the current
subdirectory path, and the latter takes the output from the former and
pre-parses them into a bit richer structure for easier handling.  They
should become a single API function that takes the current subdirectory
path and the remainder of argv[] (after parsing --options and revision
arguments from the command line) and returns an array of parsed pathspec
elements, and "magic" should become attributes of struct pathspec_item.

This patch implements only "top" magic because it can be hacked into the
system without such a refactoring.

The syntax for magic pathspec prefix is designed to be extensible yet
simple to type to invoke a simple magic like "from the top".  The parser
for the magic prefix is hooked into get_pathspec() function in this patch,
and it needs to be moved when we refactor the API.

But we have to start from somewhere.

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

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