ref-filter: factor ref_array pushing into its own function

ref-filter: factor ref_array pushing into its own function

In preparation for callers constructing their own ref_array
structs, let's move our own internal push operation into its
own function.

While we're at it, we can replace REALLOC_ARRAY() with
ALLOC_GROW(), which should give the growth operation
amortized linear complexity (as opposed to growing by one,
which is potentially quadratic, though in-place realloc
growth often makes this faster in practice).

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