Replace ",<,>,& with their respective XML entities in DAV requests

If the repo url or the user email contain XML special characters, the
remote DAV server is likely to reject the LOCK requests because the XML
is then malformed.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>