gitweb: Sort the list of forks on the summary page by age

gitweb: Sort the list of forks on the summary page by age

The list of forks on the summary page was unsorted, this just makes
them sorted by age, which seems a fair way to decide which forks are
shown before the list size cut-off (15) kicks in.

s/noheader/no_header was just to make it obvious what the parameter
affects, so all the code can be found with one grep.

pb: As suggested by Mike, I have augmented this by an additional patch
that refactors the sorting logic so that it is not tied to printing
the headers.

Signed-off-by: Mike Ralphson <mike@abacus.co.uk>
Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>