gitweb: Cleanup Git logo and Git logo target generation

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>