Add method to get commits from a PR

[rick2047]

Fixes #116. I am not sure how to test this though.

[rick2047]

I would also like to add a method which only takes PR as input. But the API needs both the repo name and PR number and the PR objects do not include the repo name.

[maleadt]

Rebased, fixed, and added a test.

I would also like to add a method which only takes PR as input. But the API needs both the repo name and PR number and the PR objects do not include the repo name.

That can be accessed as pr.repo.base. I added a method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.23%. Comparing base (9cac3b9) to head (48d810b).
Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #190   +/-   ##
=======================================
  Coverage   56.22%   56.23%           
=======================================
  Files          36       36           
  Lines         843      850    +7     
=======================================
+ Hits          474      478    +4     
- Misses        369      372    +3     

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