Ignore file(s) privately, updating .git/info/exclude file

Show/update `.gitignore` files.
Part of `git-extras`. See also `git ignore-io`.
More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-ignore>.
Trick copyright: tl;dr; <https://github.com/tldr-pages/tldr>

git ignore {{file_pattern}} --private

click the source code to copy install git ignore on any operating system with command-not-found.com