archive: load index before pathspec checks

archive: load index before pathspec checks

git archive checks whether pathspec arguments match anything to avoid
surprises due to typos and later loads the index to get attributes.

This order was OK when these features were introduced by ba053ea96c
(archive: do not read .gitattributes in working directory, 2009-04-18)
and d5f53d6d6f (archive: complain about path specs that don't match
anything, 2009-12-12).

But when attribute matching was added to pathspec in b0db704652
(pathspec: allow querying for attributes, 2017-03-13), the pathspec
checker in git archive did not support it fully, because it lacks the
attributes from the index.

Load the index earlier, before the pathspec check, to support attr
pathspecs.

Reported-by: Ronan Pigott <ronan@rjp.ie>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>