gitweb: Divide page path into directories -- path's "breadcrumbs"

gitweb: Divide page path into directories -- path's "breadcrumbs"

Divide page path into directories, so that each part of path links to
the "tree" view of the $hash_base (or HEAD, if $hash_base is not set)
version of the directory.

If the entity is blob, final part (basename) links to $hash_base or
HEAD revision of the "raw" blob ("blob_plain" view).  If the entity is
tree, link to the "tree" view.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>