Fix custom completion args for bash

[Frizlab]

I’ve noticed when using a custom completion in bash, the custom completion handler does not receive all the arguments as would be expected. This PR fixes that.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[kylemacomber]

@swift-ci please test

[kylemacomber]

@Frizlab could describe a simple concrete example where the current behavior returns the wrong result?

[Frizlab]

Here is an example! https://gitlab.com/frizlab-demo-projects/swift-argument-parser-example-for-pr-320

To see the bug, load the completion script in bash, and type ./.build/debug/example --option1 a (with a space after the a) and then trigger autocomplete. The file completion_debug.txt will be created, with one line per completion call, each line showing the content of the args array in the completion function.

You’ll see the args array only contain one entry, but it should be containing three.

With my fix, it does contain the three elements.

As a comparison one can try zsh completion, which does indeed show the three arguments as expected.

[natecook1000]

Thanks for this fix, @Frizlab! 🎉