Use a relative URL to fetch the Diffy logo

The Diffy logo is located on the Gerrit root path, which may not be the
root path of the domain.

Use a relative URL path so that the logo can be located in any mounted
location on the front-end reverse proxy.

Change-Id: Ie87819ff0eefa9a49b99665238c80428028c7015
(cherry picked from commit 1bfc2ff57843e14702a7228baea6cfc5cfe7246e)

src/main/resources/static/css/intro.css

diff --git a/src/main/resources/static/css/intro.css b/src/main/resources/static/css/intro.css
index 07269f6..04195c0 100644
--- a/src/main/resources/static/css/intro.css
+++ b/src/main/resources/static/css/intro.css
@@ -56,7 +56,7 @@
 }
 
 .header-title::before {
-    background-image: url("/diffy.png");
+    background-image: url("../../../../diffy.png");
     --icon-width: 1em;
     --icon-height: 1em;
     background-size: 1em 1em;