I hate doing this but I am tired of touching a fish script as part of
some change and having `make style` radically change it. Which makes
editing fish scripts more painful than it needs to be. It is time to do
a wholesale reformatting of these scripts to conform to the documented
style as implemented by the `fish_indent` program.
The following behaviour is added:
- an empty pushd exchanges the top two directories in the stack;
- pushd +<n> rotates the stack so that the n-th directory (counting from the left of the list shown by dirs, starting with zero) is at the top;
- pushd -<n> rotates the stack so that the nth directory (counting from the right of the list shown by dirs, starting with zero) is at the top.
