receive-pack: use bug() and BUG_if_bug()

receive-pack: use bug() and BUG_if_bug()

Amend code added in a6a84319686 (receive-pack.c: shorten the
execute_commands loop over all commands, 2015-01-07) and amended to
hard die in b6a4788586d (receive-pack.c: die instead of error in case
of possible future bug, 2015-01-07) to use the new bug() function
instead.

Let's also rename the warn_if_*() function that code is in to
BUG_if_*(), its name became outdated in b6a4788586d.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>