rerere: represent time duration in timestamp_t internally

rerere: represent time duration in timestamp_t internally

The two configuration variables, gc.rerereResolved and
gc.rerereUnresolved, are measured in days and are passed as such
into the prune_one() helper function, which worked in time_t to see
if an entry in the rerere database is past its expiry.

Instead, have the caller turn the number of days into the expiry
timestamp.  Further, use timestamp_t instead of time_t.  This will
make it possible to extend the way the configuration variable is
spelled by using date.c::parse_expiry_date() that gives the expiry
timestamp in timestamp_t.

Signed-off-by: Junio C Hamano <gitster@pobox.com>