config.mak.dev: enable -Wunused-parameter by default

Having now removed or annotated all of the unused function parameters in
our code base, I found that each instance falls into one of three
categories:

  1. ignoring the parameter is a bug (e.g., a function takes a ptr/len
     pair, but ignores the length). Detecting these helps us find the
     bugs.

  2. the parameter is unnecessary (and usually left over from a
     refactoring or earlier iteration of a patches series). Removing
     these cleans up the code.

  3. the function has to conform to a specific interface (because it's
     used via a function pointer, or matches something on the other side
     of an #ifdef). These ones are annoying, but annotating them with
     UNUSED is not too bad (especially if the compiler tells you about
     the problem promptly).

Certainly instances of (3) are more common than (1), but after finding
all of these, I think there were enough cases of (1) that it justifies
the work in annotating all of the (3)s.

And since the code base is now at a spot where we compile cleanly with
-Wunused-parameter, turning it on will make it the responsibility of
individual patch writers going forward.

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

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