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.
This is another step to resolving issue #1310. It makes
`fish_breakpoint_prompt` a replacement for `fish_prompt` if it is defined
and we're presenting a prompt in the context of a `breakpoint` command.
