ui-repolist: properly sort by age

When empty repos exist, comparing them against an existing repo with a
good mtime might, with particular qsort implementations, not sort
correctly, because of this brokenness:

   if (get_repo_modtime(r1, &t) && get_repo_modtime(r2, &t))

However, sorting by the age column works as expected, so anyway, to tidy
things up, we simply reuse that function.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>