gitweb: last-modified time should be commiter, not author

gitweb: last-modified time should be commiter, not author

The last-modified time header added by RSS to increase cache hits from
readers should be set to the date the repository was last modified. The
author time in this respect is not a good guess because the last commit
might come from a oldish patch.

Use the committer time for the last-modified header to ensure a more
correct guess of the last time the repository was modified.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>