Merge branch 'wk/gitweb-snapshot-use-if-modified-since'

Makes 'snapshot' request to "gitweb" honor If-Modified-Since: header, based on the commit date. By W. Trevor King * wk/gitweb-snapshot-use-if-modified-since: gitweb: add If-Modified-Since handling to git_snapshot(). gitweb: refactor If-Modified-Since handling gitweb: add `status` headers to git_feed() responses.