http-push: use an argv_array for setup_revisions

http-push: use an argv_array for setup_revisions

This drops the magic number for the fixed-size argv arrays,
so we do not have to wonder if we are overflowing it. We can
also drop some confusing sha1_to_hex memory allocation
(which seems to predate the ring of buffers allowing
multiple calls), and get rid of an unchecked sprintf call.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>