git-compat-util: add UNUSED macro

git-compat-util: add UNUSED macro

In preparation for compiling with -Wunused-parameter, we'd like to be
able to annotate some function parameters as false positives (e.g.,
parameters which must exist to conform to a callback interface).

Ideally our annotation will:

  - be portable, turning into nothing on platforms which don't support
    it

  - be easy to read, without looking too syntactically odd or taking
    attention away from the rest of the parameters

  - help us notice when a parameter marked as unused is actually used,
    which keeps our annotations accurate. In theory a compiler could
    tell us this easily, but gcc has no such warning. Clang has
    -Wused-but-marked-unused, but it triggers false positives with our
    MAYBE_UNUSED annotation (e.g., for commit-slab functions)

This patch introduces an UNUSED() macro which takes the parameter name
as an argument. That lets us tweak the name in such a way that we'll
notice if somebody tries to use it. It looks like this in use:

  int some_ref_cb(const char *refname,
                  const struct object_id *UNUSED(oid),
                  int UNUSED(flags),
                  void *UNUSED(data))
  {
        printf("got refname %s", refname);
        return 0;
  }

Because the unused parameter names are rewritten behind the scenes to
UNUSED_oid, etc, adding code like:

  printf("oid is %s", oid_to_hex(oid));

will fail compilation with "oid undeclared". Sadly, the "did you mean"
feature of modern compilers is not generally smart enough to suggest the
"unused" name. If we used a very short prefix like U_oid, that does
convince gcc to say "did you mean", but since the "U_" in the suggestion
isn't much of a hint, it doesn't really help. In practice, a look at the
function definition usually makes the problem pretty obvious.

Note that we have to put the definition of UNUSED early in
git-compat-util.h, because it will eventually be used for some compat
functions themselves (both directly here and in mingw.h).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>