Added action tree-parent and bound it to backspace by default.

Added action tree-parent and bound it to backspace by default.

This allow to leave the current directory in the tree view and change to
the parent tree / directory. At the top of the tree, this closes the tree
view. Bound to ',' by default.

Example:

  bind tree Left tree-parent
  bind tree Right Enter
  bind blob Left view-close

This allows to navigate the tree and blob views just with the cursor keys.

[ Cleanup the patch a bit and mention the default keybinding in
  manual.txt --jonas ]

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>