gitweb: Cleanup Git logo and Git logo target generation

Rename $githelp_url and $githelp_label to $logo_url and $logo_label to
be more obvious what they refer to; while at it add commented out
previous contents (git documentation at kernel.org). Add comment about
logo size.

Use $cgi->a(...) to generate Git logo link; it automatically escapes
attribute values when it is needed.  Escape href attribute using
esc_url instead of (incorrect!) esc_html.

Move styling of git logo <img> element from "style" attribute to CSS
via setting class to "logo".  Perhaps we should set it by id rather
than by class.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.css

@@ -16,6 +16,11 @@ a:hover, a:visited, a:active {
 	color: #880000;
 }
 
+img.logo {
+	float: right;
+	border-width: 0px;
+}
+
 div.page_header {
 	height: 25px;
 	padding: 8px;