This allows web sites to add some specific html headers to the pages
generated by gitweb.
The new variable $site_html_head_string can be set to an html snippet that
will be inserted at the end of the <head> section of each page generated
by gitweb.
Signed-off-by: Lénaïc Huard <redacted>
Signed-off-by: Junio C Hamano <redacted>