remote-curl: use argv_array in parse_push()

remote-curl: use argv_array in parse_push()

Use argv_array to build an array of strings instead of open-coding it.
This simplifies the code a bit.

We also need to make the specs parameter of push(), push_dav() and
push_git() const to match the argv member of the argv_array.  That's
fine, as all three only actually read from the specs array anyway.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>