Revert most of "Fix linting errors that snuck in."

[gaelicWizard]

Description

This reverts most of commit 2c8ee40.

Motivation and Context

• Shellcheck documentation for the source-path directive indicates this is correct usage. We're sourceing the bash-preexec.sh file from inside the vendor/github.com/rcaloras/bash-preexec directory. If we used the source directive, then the full complete path to the file itself would need to be specified.
• Fix disable=1090 to disable=SC1090 and remove duplicate lines since this shellcheck directive will apply to the entire if-ladder.
• Disabling SC2154 is almost never appropriate. In this case, several _git_bash_completion* variables are expressly assigned in this file, so there is no "unknown" to ignore.

Aside: the ${!_git_bash_completion@} construct will expand to all variables starting with the prefix _git_bash_completion, so this line is just a shorthand way to clear all our variables concisely without forgetting any.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[gaelicWizard]

@cornfeedhobo, you may have an old or broken shellcheck on your system! ♥

[NoahGorny]

Hmm, seems like we should always check with the newest shellcheck (or the one we use in the CI at least)