Support fish correct completion for nested subcommands

[ModProg]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

master

Describe your use case

There are two cases where the current solution does not work:

Simple case (can be solved with fish build-ins)

Example command syntax:
command A B <parameter>
command A C <parameter>
Currently pressing tab after writing A it will complete B as well as C again.

Complex case (we probably would need to write something our own)

Probably wont happen that often:
Two subcommand chains:
command A B C
command B A D
These are even with the simple solution not differentiable.

Describe the solution you'd like

Simple Case

Instead of only requirering the parent subcommand to exist, rather also check that self and the sibling commands do not exist:

complete -c command -n "__fish_use_subcommand" -x -a "A"
complete -c command -n "__fish_seen_subcommand_from A; and not __fish_seen_subcommand_from B C" -x -a "B"
complete -c command -n "__fish_seen_subcommand_from A; and not __fish_seen_subcommand_from B C" -x -a "C"

Complex case

Not sure how to solve. Probably need something like

clap/src/completions.rs

Lines 237 to 248 in a61eaf8

	r#"function __fish_using_command
	set cmd (commandline -opc)
	if [ (count $cmd) -eq (count $argv) ]
	for i in (seq (count $argv))
	if [ $cmd[$i] != $argv[$i] ]
	return 1
	end
	end
	return 0
	end
	return 1
	end

again.
With fish buildins A B and B A is the same. Therefor you would get both C and D for both. The same would happen if at some point a subcommand gets repeated (maybe more likely than the AB BA example).

Insufficient solution using standard complete functionality

complete -c command -n "__fish_use_subcommand" -x -a "A"
complete -c command -n "__fish_use_subcommand" -x -a "B"
complete -c command -n "__fish_seen_subcommand_from A; and not __fish_seen_subcommand_from B" -x -a "B"
complete -c command -n "__fish_seen_subcommand_from B; and not __fish_seen_subcommand_from A" -x -a "A"
complete -c command -n "__fish_seen_subcommand_from A; and __fish_seen_subcommand_from B;  and not __fish_seen_subcommand_from C" -x -a "C"
complete -c command -n "__fish_seen_subcommand_from B; and __fish_seen_subcommand_from A;  and not __fish_seen_subcommand_from D" -x -a "D"

Alternatives, if applicable

No response

Additional Context

This is somewhat a regression see #685

[ModProg]

Hmm, maybe this would have been rather a bug than a feature

[pksunkara]

I wouldn't worry about subcommand chains since they are not supported in clap.

[ModProg]

I wouldn't worry about subcommand chains since they are not supported in clap.

I did not mean chained commands, I meant multiple level of subcommands, where the naming is repeating.

[ModProg]

I'll have a look at how the simple version can be implemented.