Make pages inherit their sidebar/header/footer from parent directories regardless of the current 'page_file_dir' of the wiki. refs #413 (https://github.com/github/gollum/issues/413)

lib/gollum/page.rb

@@ -434,7 +434,9 @@ module Gollum
     end
 
     # Loads a sub page.  Sub page names (footers, headers, sidebars) are prefixed with
-    # an underscore to distinguish them from other Pages.
+    # an underscore to distinguish them from other Pages. If there is not one within
+    # the current directory, starts walking up the directory tree to try and find one
+    # within parent directories.
     #
     # name - String page name.
     #
@@ -447,7 +449,7 @@ module Gollum
 
       dirs = self.path.split('/')
       dirs.pop
-      map = @wiki.tree_map_for(@wiki.ref)
+      map = @wiki.tree_map_for(@wiki.ref, true)
       while !dirs.empty?
         if page = find_page_in_tree(map, name, dirs.join('/'))
           page.parent_page = self