Add and update commit frequency metrics

[d33bs]

Description

This PR adds metrics surrounding commits and commit frequency. Along the journey towards this work I found that some functionality wasn't working as expected (or maybe wasn't labeled clearly in the context of these changes). As a result I took time to fix things and evolve the work towards consistency. This also included making modifications to a few tests and the repo_setup function, which is becoming more and more important to creating and implementing tests for these changes.

For commit frequency I used commits per day as a calculation to help avoid inconsistencies when it comes to weeks, months and years. I could also see how it might be better to perform a different type of calculation here, let me know if something seems more useful here.

Closes #157
Closes #158

What is the nature of your change?

• Content additions or updates (adds or updates content)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own contributions.
• I have commented my content, particularly in hard-to-understand areas.
• I have made corresponding changes to related documentation (outside of book content).
• My changes generate no new warnings.
• New and existing tests pass locally with my changes.
• I have added tests that prove my additions are effective or that my feature works.
• I have deleted all non-relevant text in this pull request template.

[falquaddoomi]

This all looks really neat; kudos! I left two comments, only one of which (the count_files() one) I think needs to be considered.

[d33bs]

Thank you @gwaybio and @falquaddoomi ! Merging this in after making a few changes.