Fix executable path when calling custom completion in bash

[Frizlab]

In Bash, when calling completion for an executable written with full path (e.g. /opt/brew/bin/example), the completion script might call another executable if another executable with the same name happens to be first in the PATH. (This is particularly annoying when debugging completion in a project!) 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

[natecook1000]

Looks solid @Frizlab — can you fix up the conflicts from your other fix?

[Frizlab]

@natecook1000 Done

[natecook1000]

@swift-ci Please test

[Frizlab]

Fixed the test that was failing.

[natecook1000]

@swift-ci Please test