Add @rushstack/prettier-git

[dmichon-msft]

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Summary

Implements a wrapper around prettier that formats either staged files, or files changed since a specified Git revision. Can also be run in CI mode with --check to error if any files modified since a specified Git revision do not match the formatting produced by prettier, in which case the diff will be rendered for each impacted file.

Details

This PR is parked here as a reference, I don't currently have plans to finish the cleanup work and get it checked in any time soon.

How it was tested

Impacted documentation

[octogonz]

CC @elliot-nelson

[elliot-nelson]

It looks like this was already in motion -- just curious, what's the main difference between this tool and pretty-quick that motivated creating the new wrapper? (I guess question for @dmichon-msft )

[dmichon-msft]

I wrote the code here a couple years ago now. The main motivations:

1. Support for partially staged files. If you have a file with both staged and unstaged changes, rather than erroring out like pretty-quick does, it will format the staged version and write the result directly to the index, so that you can keep your unstaged changes unstaged.
2. More customizability with respect to which files to operate on, namely the ability to format all files modified since some arbitrary Git revision