footer: link back to cgit home page

The footer has always been overrideable using the footer= in cgitrc, so
this won't anger anybody who cares about their footer.

cgit.css

@@ -590,6 +590,13 @@ div#cgit div.footer {
 	font-size: 80%;
 	color: #ccc;
 }
+div#cgit div.footer a {
+	color: #ccc;
+	text-decoration: none;
+}
+div#cgit div.footer a:hover {
+	text-decoration: underline;
+}
 div#cgit a.branch-deco {
 	color: #000;
 	margin: 0px 0.5em;

ui-shared.c

@@ -740,7 +740,7 @@ void cgit_print_docend()
 	if (ctx.cfg.footer)
 		html_include(ctx.cfg.footer);
 	else {
-		htmlf("<div class='footer'>generated  by cgit %s at ",
+		htmlf("<div class='footer'>generated by <a href='http://git.zx2c4.com/cgit/about/'>cgit %s</a> at ",
 			cgit_version);
 		cgit_print_date(time(NULL), FMT_LONGDATE, ctx.cfg.local_time);
 		html("</div>\n");