Update Tests Fetcher

[DN6]

What does this PR do?

Tests fetcher on PRs is currently running all tests in Github actions due to how GA performs checkout. The Github Actions checkout does not have a local refs to main (which is what test fetcher attempts to use to diff the PR and find the changes).

This PR

1. Changes it so tests fetcher diffs the PR with the the upstream main branch to correctly figure out the tests.
2. Adds support to run the tests for Core Pipelines for every PR in a dedicated job
3. Changes the logic so that when tests fetcher has to run all tests in the suite, it will filter out the pipelines to run core pipeline tests only, instead of testing every pipeline.
4. Updates the GitPython version in the deps table to match the version in transformers for parity.

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[patrickvonplaten]

Ok to merge once quality tests pass

[DN6]

Failing test related to convert_kandinksy file being in the wrong location. Will open a new PR to move that.