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.
Presently, `isatty` only works on a handful of keywords. Here it
is rewritten to be able to take any path, device or fd number as
an argument, and eliminates errors printed to stdout.
Per discussion in #1228, using `builtin test -c` within a pipe to
test special file descriptors is not viable, so this implementation
specifcially uses `command test`. Additionally, a note has been
added to the documentation of `test` regarding this potential
aberration from the expected output of the test utility under the
'Standards' section.
