Prior to this fix, __fish_describe_command would error if the
input contained any special characters, because it would be interpolated
into a regex. Hack in a guard to do nothing if the input contains
anything other than [a-zA-Z0-9_ ]
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.
