gitweb: Fix "Use of unitialized value" warnings in empty repository

gitweb: Fix "Use of unitialized value" warnings in empty repository

Fix it so gitweb doesn't write "Use of unitialized value..." warnings
(which gets written in web server logs) for empty (no commits)
repository.

In empty repository "last change" (last activity) doesn't make sense;
also there is no sense in parsing commits which aren't there.

In projects list for empty repositories gitweb now writes "No commits"
using "noage" class, instead of leaving cell empty, in the last change
column.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>