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>

builtin/name-rev.c

@@ -15,7 +15,7 @@ typedef struct rev_name {
 	int distance;
 } rev_name;
 
-static long cutoff = LONG_MAX;
+static timestamp_t cutoff = TIME_MAX;
 
 /* How many generations are maximally preferred over _one_ merge traversal? */
 #define MERGE_TRAVERSAL_WEIGHT 65535