filter: fix libravatar email-filter https issue

Serving cgit via https and getting avatar via http gives error messages
about untrusted content. This decides whether or not to use https link
by looking at the environment variable HTTPS, which is set in CGI.

filters/email-libravatar.lua

@@ -15,7 +15,8 @@ function filter_open(email, page)
 end
 
 function filter_close()
-	html("<img src='//cdn.libravatar.org/avatar/" .. md5 .. "?s=13&amp;d=retro' width='13' height='13' alt='Libravatar' /> " .. buffer)
+	baseurl = os.getenv("HTTPS") and "https://seccdn.libravatar.org/" or "http://cdn.libravatar.org/"
+	html("<img src='" .. baseurl .. "avatar/" .. md5 .. "?s=13&amp;d=retro' width='13' height='13' alt='Libravatar' /> " .. buffer)
 	return 0
 end