Fix: repo-config for pullRequests.draft should not be wiped when combined with _unset_ config

[rtyley]

This fixes a bug that came in with the initial introduction of the pullRequests.draft config setting:

• Allow configuring Pull Requests to be 'draft' #3628

The bug was that when combining global & local repo config, the result of combining pullRequests.draft = true with a config with no setting for pullRequests.draft would be that pullRequests.draft was unset (ie given the value None, rather than Some(true).

Here's an example of where this was a problem in the wild:

• Use Scala script for targeted releases guardian/scala-steward-public-repos#102 (comment)

Cause

This arose because of the behaviour of .mapN(), which I hadn't really thought through:

scala-steward/modules/core/src/main/scala/org/scalasteward/core/repoconfig/PullRequestsConfig.scala

Line 66 in 373d5a7

draft = (x.draft, y.draft).mapN(_ || _)

...in retrospect, it's not too surprising that .mapN() on two Options only produces a populated Some if both the inputs are Some - which is not the behaviour we want here. For us, if only one of the configs has a value set, we definitely want to use that value.

Fix

In the end, the neatest way I could find to get the desired behaviour was to remove the thing stopping us from having a Monoid[Option[Boolean]] - ie that there is no general Monoid[Boolean] (there is no one correct way to combine two Booleans). We can create a specific one for this case, that says Booleans should be combined with logical-OR (rather than AND, for instance) - and only expose it in the very narrow scope of where we want to use it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.77%. Comparing base (7e9cf7f) to head (6069792).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3719   +/-   ##
=======================================
  Coverage   89.77%   89.77%           
=======================================
  Files         174      174           
  Lines        5054     5055    +1     
  Branches      489      492    +3     
=======================================
+ Hits         4537     4538    +1     
  Misses        517      517           

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