gitweb: check if-modified-since for feeds

Offering Last-modified header for feeds is only half the work, even if
we bail out early on HEAD requests. We should also check that same date
against If-modified-since, and bail out early with 304 Not Modified if
that's the case.

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

gitweb/gitweb.perl

@@ -6015,7 +6015,25 @@ sub git_feed {
 	}
 	if (defined($commitlist[0])) {
 		%latest_commit = %{$commitlist[0]};
-		%latest_date   = parse_date($latest_commit{'committer_epoch'});
+		my $latest_epoch = $latest_commit{'committer_epoch'};
+		%latest_date   = parse_date($latest_epoch);
+		my $if_modified = $cgi->http('IF_MODIFIED_SINCE');
+		if (defined $if_modified) {
+			my $since;
+			if (eval { require HTTP::Date; 1; }) {
+				$since = HTTP::Date::str2time($if_modified);
+			} elsif (eval { require Time::ParseDate; 1; }) {
+				$since = Time::ParseDate::parsedate($if_modified, GMT => 1);
+			}
+			if (defined $since && $latest_epoch <= $since) {
+				print $cgi->header(
+					-type => $content_type,
+					-charset => 'utf-8',
+					-last_modified => $latest_date{'rfc2822'},
+					-status => '304 Not Modified');
+				return;
+			}
+		}
 		print $cgi->header(
 			-type => $content_type,
 			-charset => 'utf-8',