Fix wrong time_t used in function

This function doesn't interoperate with any system functions that
use the system time_t, but rather only works with the git_time_t
type in libgit2, which could be a different width than the system
one. Fixes a compile warning.

src/date.c

@@ -204,7 +204,7 @@ static int is_date(int year, int month, int day, struct tm *now_tm, time_t now,
 	if (month > 0 && month < 13 && day > 0 && day < 32) {
 		struct tm check = *tm;
 		struct tm *r = (now_tm ? &check : tm);
-		time_t specified;
+		git_time_t specified;
 
 		r->tm_mon = month - 1;
 		r->tm_mday = day;