feat: upgrade go-git

[caarlos0]

Description

https://github.com/src-d/go-git was deprecated, the new repo is https://github.com/go-git/go-git.

This PR upgrades from the older to the new.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Examples of user facing changes:
- upgrades go-git from v4 to v5.

[caarlos0]

I would also say maybe delete the vendor folder and use just go.mod and go.sum, so the diffs are nicer? 🤔

[tejal29]

I would also say maybe delete the vendor folder and use just go.mod and go.sum, so the diffs are nicer? 🤔

The reason we use vendor mode is make sure travis jobs don't download all the dependencies and have faster run time.