refs.h: let `for_each_namespaced_ref()` take excluded patterns

refs.h: let `for_each_namespaced_ref()` take excluded patterns

A future commit will want to call `for_each_namespaced_ref()` with
a list of excluded patterns.

We could introduce a variant of that function, say,
`for_each_namespaced_ref_exclude()` which takes the extra parameter, and
reimplement the original function in terms of that. But all but one
caller (in `http-backend.c`) will supply the new parameter, so add the
new parameter to `for_each_namespaced_ref()` itself instead of
introducing a new function.

For now, supply NULL for the list of excluded patterns at all callers to
avoid changing behavior, which we will do in a future change.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>