(OutputAlreadyTrackedError): Add git suggestions to stop tracking files

[PuneethaPai]

Fixes: #3678

When file has been already tracked under git, OutputAlreadyTrackedError gets raised.
This commits add git command suggestions to help user to stop tracking file from git,
so in future commits they can use DVC for the file.

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

• ❌ I will check DeepSource, CodeClimate, and other sanity checks below. (We consider them recommendatory and don't expect everything to be addressed. Please fix things that actually improve code or fix bugs.)

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[pared]

LGTM, though I would keep original issue open (or create new one), as we did not discuss whether we
should include suggestions about https://rtyley.github.io/bfg-repo-cleaner/

[efiop]

@pared Let's not ecommend bfg for now, as it is too specific. Merging and closing the original issue.

Thanks! @PuneethaPai 🙏

[shcheklein]

@PuneethaPai also, it looks there is something funny happening with your Git settings - Github does not attribute commits properly to you (probably some discrepancy in emails - Github and local Git?)

[PuneethaPai]

Thanks @shcheklein and @efiop . I just realized my local git user.email was configured with different id than the one with github login. Thanks for pointing this out. Will keep in mind in future.
PS: Big fan of this project. 🥰

[efiop]

@PuneethaPai You can still add it in https://github.com/settings/emails , so github will automatically associate your existing commits with your profile.