git-compat-util: introduce ASSERT() macro

git-compat-util: introduce ASSERT() macro

Create a ASSERT() macro which is similar to assert(), but will not be
compiled out when NDEBUG is defined, and is thus safe to use even if its
argument has side-effects.

We will use this new macro in a subsequent commit to convert a few
existing assert() invocations to ASSERT().  In particular, we'll
convert the handful of invocations which cannot be proven to be free of
side effects with a simple compiler/linker hack.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>