Auto merge of #76356 - caass:hooks, r=jyn514

Add a command to install a git hook to automatically run `x.py test tidy --bless`

Some folks (such as myself) would probably find a lot of convenience in a pre-commit hook that automatically runs tidy before committing, to avoid burning CI time learning that your commit wasn't tidy.

I'm absolutely positive I have missed some stuff. I basically just got this to where you can run `./x.py run install-git-hook` and then clicked the commit button. Please let me know what else you'd like me to add before this can be merged!

[rustc-dev-guide companion PR](rust-lang/rustc-dev-guide#848)

Author: bors