t0006 & t5000: prepare for 64-bit timestamps

t0006 & t5000: prepare for 64-bit timestamps

Git's source code refers to timestamps as unsigned longs. On 32-bit
platforms, as well as on Windows, unsigned long is not large enough to
capture dates that are "absurdly far in the future".

It is perfectly valid by the C standard, of course, for the `long` data
type to refer to 32-bit integers. That is why the `time_t` data type
exists: so that it can be 64-bit even if `long` is 32-bit. Git's source
code simply uses an incorrect data type for timestamps, is all.

The earlier quick fix 6b9c38e14cd (t0006: skip "far in the future" test
when unsigned long is not long enough, 2016-07-11) papered over this
issue simply by skipping the respective test cases on platforms where
they would fail due to the data type in use.

This quick fix, however, tests for *long* to be 64-bit or not. What we
need, though, is a test that says whether *whatever data type we use for
timestamps* is 64-bit or not.

The same quick fix was used to handle the similar problem where Git's
source code uses `unsigned long` to represent size, instead of `size_t`,
conflating the two issues.

So let's just add another prerequisite to test specifically whether
timestamps are represented by a 64-bit data type or not. Later, after we
switch to a larger data type, we can flip that prerequisite to test
`time_t` instead of `long`.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>