Get all directory entries at the given SHA

This commit adds Gollum-formatted entries that represent the directory
tree of the repository at the given SHA. For example, using the Gollum
`lotr.git` test wiki, we get the following array back:

    @repo.lstree_all_directories(sha)
    =>
    [{:sha=>"6e48abfc56565574859e081ee58eae655d48cf71",
      :mode=>16384,
      :type=>"tree",
      :name=>"Gondor",
      :path=>"Gondor"},
     {:sha=>"760982a3b84987919b99748d87c7890bb54afd07",
      :mode=>16384,
      :type=>"tree",
      :name=>"Mordor",
      :path=>"Mordor"},
     {:sha=>"a13e77aca82edd8e6bd4096e351627859f74ffec",
      :mode=>16384,
      :type=>"tree",
      :name=>"Rivendell",
      :path=>"Rivendell"}]

In subsequent commits to `gollum` and `gollum-lib`, using this method
can help us make Gollum's "Overview" page more performant for large
wikis.

Currently, Gollum has no mechanism for getting the directory tree of
the git repository, meaning we have to reverse-engineer the directory
tree by using the paths of files checked into the repository.

lib/rugged_adapter/git_layer_rugged.rb

@@ -659,6 +659,12 @@ def lstree(sha, options = {})
         results
       end
 
+      def lstree_all_directories(sha)
+        @repo.lookup(sha).tree.each_tree.map { |rugged_tree_entry|
+          ::Gollum::Git::Tree.tree_entry_from_rugged_hash rugged_tree_entry
+        }
+      end
+
       def path
         @repo.path
       end