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>