decorate: add clear_decoration() function

decorate: add clear_decoration() function

There's not currently any way to free the resources associated with a
decoration struct. As a result, we have several memory leaks which
cannot easily be plugged.

Let's add a "clear" function and make use of it in the example code of
t9004. This removes the only leak from that script, so we can mark it as
passing the leak sanitizer.

Curiously this leak is found only when running SANITIZE=leak with clang,
but not with gcc.  But it is a bog-standard leak: we allocate some
memory in a local variable struct, and then exit main() without
releasing it. I'm not sure why gcc doesn't find it. After this
patch, both compilers report it as leak-free.

Note that the clear function takes a callback to free the individual
entries. That's not needed for our example (which is just decorating
with ints), but will be for real callers.

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