gitweb: Send XHTML as 'application/xhtml+xml' where possible

"The 'text/html' media type [RFC2854] is primarily for HTML, not for
XHTML. In general, this media type is NOT suitable for XHTML."

This patch makes gitweb use content negotiation to conservatively send
pages as Content-Type 'application/xhtml+xml' when the user agent
explicitly claims to support it.

It falls back to 'text/html' even if the user agent appears to
implicitly support 'application/xhtml+xml' due to a '*/*' glob, working
around an insidious bug in Internet Explorer where sending the correct
media type prevents the page from being displayed.

Signed-off-by: Alp Toker <alp@atoker.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/gitweb/gitweb.cgi b/gitweb/gitweb.cgi
index 2e87de4..bd9b9de 100755
--- a/gitweb/gitweb.cgi
+++ b/gitweb/gitweb.cgi
@@ -293,7 +293,17 @@
 			}
 		}
 	}
-	print $cgi->header(-type=>'text/html',  -charset => 'utf-8', -status=> $status, -expires => $expires);
+	my $content_type;
+	# require explicit support from the UA if we are to send the page as
+	# 'application/xhtml+xml', otherwise send it as plain old 'text/html'.
+	# we have to do this because MSIE sometimes globs '*/*', pretending to
+	# support xhtml+xml but choking when it gets what it asked for.
+	if ($cgi->http('HTTP_ACCEPT') =~ m/(,|;|\s|^)application\/xhtml\+xml(,|;|\s|$)/ && $cgi->Accept('application/xhtml+xml') != 0) {
+		$content_type = 'application/xhtml+xml';
+	} else {
+		$content_type = 'text/html';
+	}
+	print $cgi->header(-type=>$content_type,  -charset => 'utf-8', -status=> $status, -expires => $expires);
 	print <<EOF;
 <?xml version="1.0" encoding="utf-8"?>
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
@@ -301,7 +311,7 @@
 <!-- git web interface v$version, (C) 2005-2006, Kay Sievers <kay.sievers\@vrfy.org>, Christian Gierke -->
 <!-- git core binaries version $git_version -->
 <head>
-<meta http-equiv="content-type" content="text/html; charset=utf-8"/>
+<meta http-equiv="content-type" content="$content_type; charset=utf-8"/>
 <meta name="robots" content="index, nofollow"/>
 <title>$title</title>
 <link rel="stylesheet" type="text/css" href="$stylesheet"/>