Akin to __fish_complete_suffix, __fish_complete_directories now attempts
to complete the current commandline token if no token is explicitly
passed in as an argument.
Reviewing a PR for a completion script caused me to look at the
implementation for the `__fish_complete_directories` function. Which in
turn lead me to create this change to improve its implementation and add
unit tests for the function.
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.
