I'm going to modify these functions as part of dealing with issue #3000
and don't want those changes to be masked by running the files through
`make style`.
The existing implementation grows the $dirprev array without bounds. Besides
causing what would appear to be a memory leak it also makes the nextd and
prevd commands more expensive than they need to be. It also makes it harder to
create a useful "menu" cd command.
In addition to implementing a reasonable limit on the size of the $dirprev
array I've reformatted the code using fish_indent.
Update the documentation to include mentions of the $dirprev and $dirnext
variables as well as the limit on how much directory history is kept.
Fixes 2836
