Using bare vars is more efficient because it makes the builtin `math`
expression cache more useful. That's because if you prefix each var with
a dollar-sign then the fish parser expands it before `math` is run.
Something like `math x + 1` can be cached since the expression is the
same each time it is run. But if you do `math $x + 1` and x==1 then you're
effectively executing `math 1 + 1`. And if x==2 the next time then you're
running `math 2 + 1`. Which makes the expression cache much less effective.
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.
