gitweb: make logo optional
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 <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Acked-by: Jakub Narebski <a6a1517a087f9f64a72f41a0de57ff01fbb0e412@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git