Clean up docs

[unixorn]

Description

• Make git-root-directory, git-ignored and git-nuke-untracked-files return useful exit codes
• Clean quoting; remove extra space in command
• Quote bare variables in git-comma
• Update doc in git-upstream-sync

Type of changes

• A helper script
• script cleanups
• A link to an external resource like a blog post

Checklist:

• All new and existing tests pass.
• Any scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh is an ok exception)\
• Scripts are marked executable
• I have confirmed that the link(s) in my PR are valid.
• I have read the CONTRIBUTING document.