Add completion for prime-run

[oriko1010]

Description

Simple completion for the prime-run script.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[krobelus]

$args should be something like (commandline -opc)[2..], or else it will only complete the command name - try
prime-run grep -- and press Tab. (Though this doesn't seem terribly important here.)

[oriko1010]

$ prime-run grep --[TAB]
--after-context                          (Print NUM lines of trailing context)
--basic-regexp                                        (Pattern is basic regex)
...

Unless you're referring to something else, this does seem to work for me. I used the sudo completion as a guide. Though now I realized the $args probably isn't even necessary, as prime-run has no arguments of its own.

[krobelus]

Oh I see, this happens to work because

• $args is undefined here, so it is an empty list (the parameters to a function are in $argv)
• In __fish_complete_subcommand.fish, $subcommand is set to the empty list.
• As a result the subcommand to complete is based off (commandline -cop) (commandline -ct) which is the current command line left of the cursor.

Yep, the --commandline option is useful for passing a custom commandline when completing commands like sudo that accept options on their own.
Merged as 8b02a78 with the simplification in 46a69d6, thanks!