Add parameters to configure merge commit messages to repo

[garrettheel]

These new parameters have been added to the repository api to allow more granular control of merge commit messages:

• squash_merge_commit_title
• squash_merge_commit_message
• merge_commit_title
• merge_commit_message

[codecov]

Codecov Report

Merging #2440 (0fe7b9f) into master (2eccd8c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2440   +/-   ##
=======================================
  Coverage   98.08%   98.08%           
=======================================
  Files         122      122           
  Lines       10632    10636    +4     
=======================================
+ Hits        10428    10432    +4     
  Misses        140      140           
  Partials       64       64           

Impacted Files	Coverage Δ
github/repos.go	98.67% <100.00%> (+<0.01%)	⬆️

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[gmlewis]

Thank you, @garrettheel !
LGTM.
Merging.

[gmlewis]

Looking at the linked docs, these 4 new fields have a small number of allowed values...
I think it might be beneficial to include them as comments, such as:

// SquashMergeCommitTitle can be one of: "PR_TITLE", "COMMIT_OR_PR_TITLE"

Thoughts?

[garrettheel]

Looking at the linked docs, these 4 new fields have a small number of allowed values... I think it might be beneficial to include them as comments

@gmlewis yep, that makes sense! I wasn't sure how much we wanted to re-document the API here, but I think this would help quickly figure out valid inputs. Pushed up another commit with comments, let me know if that's the right spot for them