The external-commands-only completion was briefly added in 3.1.0 and removed
in 3.1.1 (see #6798), which means we can remove some dead code.
Maybe we should just remove __fish_complete_external_command - it could break
users, but then again, we don't really have a way to stop people from starting
to use this deprecated function. The underscores ought to communicate that
this is function is private to fish but that is not enforced.
Presently the completion engine ignores builtins that are part of the
fish syntax. This can be a problem when completing a string that was
based on the output of `commandline -p`. This changes completions to
treat these builtins like any other command.
This also disables generic (filename) completion inside comments and
after strings that do not tokenize.
Additionally, comments are stripped off the output of `commandline -p`.
Fixes#5415Fixes#2705