fix: Single commit validation does not factor in merge commits [#108]

[julianpoy]

See julianpoy#2 for validation that this works.

Continuation of #127

The problem with the original PR was that GitHub API returns a list of commits each with a property commit inside. When mapping over commits => commit, I forgot to add an additional chain for commits => commit.commit.

I also noticed that validatePrTitle should check the nonMergeCommit that we found in the case that there is, in fact, only one commit, rather than any one of the commits in case the first commit returned is a merge commit (for some strange reason).

Lastly, I noticed that the test workflows included in this repository (the one titled current branch) don't use any of the additional options that this repo has to offer, such as the validateSingleCommit option. I would recommend that the current branch workflow test for this repository either include these options, or that there be multiple workflows defined - one for each option.