ref-filter: avoid using `unsigned long` for catch-all data type

ref-filter: avoid using `unsigned long` for catch-all data type

In its `atom_value` struct, the ref-filter source code wants to store
different values in a field called `ul` (for `unsigned long`), e.g.
timestamps.

However, as we are about to switch the data type of timestamps away from
`unsigned long` (because it may be 32-bit even when `time_t` is 64-bit),
that data type is not large enough.

Simply change that field to use `uintmax_t` instead.

This patch is a bit larger than the mere change of the data type
because the field's name was tied to its data type, which has been fixed
at the same time.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>