add support for `NO_COLOR` environment variable

[LucasLarson]

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

• gunstage/bin/git-unstage

  Line 31 in f172b48

  command git status

• gunstage/bin/git-unstage

  Line 43 in f172b48

  done && command git status

This repository should adopt the closest existing standard – no-color.org’s¹ NO_COLOR environmental variable – to prevent color output on demand until git(1) itself supports it.²

Footnotes

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) ↩