t0006: rename test-date's "show" to "relative"

t0006: rename test-date's "show" to "relative"

The "show" tests are really only checking relative formats;
we should make that more clear.

This also frees up the "show" name to later check other
formats. We could later fold "relative" into a more generic
"show" command, but it's not worth it.  Relative times are a
special case already because we have to munge the concept of
"now" in our tests.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>