gitweb: Sort projects with undefined ages last

Sorting gitweb's project list by age ('Last Change') currently shows
projects with undefined ages at the head of the list. This gives a less
useful result when there are a number of projects that are missing or
otherwise faulty and one is trying to see what projects have been
updated recently.

Fix by sorting these projects with undefined ages at the bottom of the
list when sorting by age.

Signed-off-by: Matthew Daley <redacted>
Signed-off-by: Junio C Hamano <redacted>