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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>