gitweb: provide a routine to display (sub)sections

The routine puts the given contento into a DIV element, automatically
adding a header div. The content can be provided as a standard scalar
value (which is used as-is), as a scalar ref (which is HTML-escaped), as
a function reference to be executed, or as a file handle to be dumped.

Signed-off-by: Giuseppe Bilotta <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>