ui-refs: simplify cmp_age logic

The check in parse_user that eventually makes it into committer_date and
tagger_date is:

else if (mode == 3 && isdigit(*p)) {
    *date = atol(p);
    mode++;
}

Since isdigit('-') is always false, date will never be negative. Thus
the sign of this function:

static int cmp_age(int age1, int age2)
{
    if (age1 != 0 && age2 != 0)
        return age2 - age1;

    if (age1 == 0 && age2 == 0)
        return 0;

    if (age1 == 0)
        return +1;

    return -1;
}

Will always be the same as the sign of this function:

static inline int cmp_age(int age1, int age2)
{
    return age2 - age1;
}

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Idea-by: Lukas Fleischer <cgit@cryptocrack.de>