never fallback relative times to absolute

never fallback relative times to absolute

Previously, for dates older than 12 months we fell back to just giving the
absolute time.  This can be a bit jarring when reading a list of times.

Instead, let's switch to "Y years, M months" for five years, and then just
"Y years" after that.

No particular reason on the 5 year cutoff except that it seemed reasonable
to me.

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