gitweb: place links to parent directories in page header

Change html page headers to not only link the project root and the
currently selected project but also the directories in between using
project_filter. (Allowing to jump to a list of all projects within
that intermediate directory directly and making the project_filter
feature visible to users).

Signed-off-by: Bernhard R. Link <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>