cogs.util.github: Show merged PRs for multi-commit pushes

[prgmitchell]

Description

Show the matching merged PR for pushes containing multiple commits.

Motivation and Context

When multi-commit PRs get merged it makes more sense to see the PR itself rather than the just the last commit.

How Has This Been Tested?

It really hasn't, but seems good to me.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Warchamp7]

I think at this point it makes more sense to just split get_commit_messages into two different functions instead of having the brief flag (And the brief version can return the results of the 'full' method if it's not a PR that needs to be shortened)

I defer to you on whether you agree and wish to perform that refactor though @prgmitchell. If not, I am fine to merge this as-is.

[prgmitchell]

@Warchamp7 although I wanted to say just merge as-is, I agree splitting makes sense and know if it isn't done now it probably won't ever be. PR updated, let me know what you think.