gitweb: make logo optional

Some sites may not want to have a logo at all.

While at it, use $cgi->img to simplify this code.  (CGI.pm learned
most HTML4 tags by version 2.79, so this should be portable to perl
5.8, though I haven't tested.)

Signed-off-by: Jonathan Nieder <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>