add support for `NO_COLOR` environment variable

[LucasLarson]

Where git unstage will return a git status, there is no way to prevent color output:

• https://github.com/LucasLarson/gunstage/blob/f172b485fda43daf264eb95c066750f518eb5bc0/bin/git-unstage#L31
• https://github.com/LucasLarson/gunstage/blob/f172b485fda43daf264eb95c066750f518eb5bc0/bin/git-unstage#L43

This repository should adopt the closest existing standard – no-color.org’s[^1] NO_COLOR environmental variable – to prevent color output on demand until git(1) itself supports it.[^2]

[^1]: jcs/no_color generates no-color.org [^2]: As of November 2023, Git’s only test for $NO_COLOR is in t/chainlint.pl, where it was added in September 2022 (git/git@7c04aa7390)