gitweb: Refactor checking if part of project info need filling

Extract the check if given keys (given parts) of project info needs to
be filled into project_info_needs_filling() subroutine.  It is for now
a thin wrapper around "!exists $project_info->{$key}".

Note that !defined was replaced by more correct !exists.

While at it uniquify treating of all project info, adding checks for
'age' field before running git_get_last_activity(), and also checking
for all keys filled in code protected by conditional, and not only
one.

The code now looks like this

  foreach my $project (@$project_list) {
  	if (given keys need to be filled) {
  		fill given keys
  	}
  	...
  }

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>