negative-refspec: fix segfault on : refspec

negative-refspec: fix segfault on : refspec

The logic added to check for negative pathspec match by c0192df630
(refspec: add support for negative refspecs, 2020-09-30) looks at
refspec->src assuming it is never NULL, however when
remote.origin.push is set to ":", then refspec->src is NULL,
causing a segfault within strcmp.

Tell git to handle matching refspec by adding the needle to the
set of positively matched refspecs, since matching ":" refspecs
match anything as src.

Add test for matching refspec pushes fetch-negative-refspec
both individually and in combination with a negative refspec.

Signed-off-by: Nipunn Koorapati <nipunn@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>