name-rev: change a "long" variable to timestamp_t

name-rev: change a "long" variable to timestamp_t

Earlier dddbad72 ("timestamp_t: a new data type for timestamps",
2017-04-26) updated all in-core variables, fields and function
return values that are used to store "seconds since epoch" to a new
type timestamp_t.  Unfortunately one variable "cutoff", which is
used to keep track of the oldest timestamp of commit we saw on the
command line, was "long" and left behind.

Update it to timestamp_t as well.

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