(maint) Replace triple dots in TRAVIS_COMMIT_RANGE with double dots

The TRAVIS_COMMIT_RANGE environment variable uses triple dots to describe the
range[1]. When that is used in `git log base...HEAD`, then it will include all
commits that are not in both A and B. That means we will include commits in the
base branch that are unrelated to the PR.

For now, do what puppet does to convert triple dots to double dots. In the
future, use GITHUB_BASE_REF to determine the range of commits.

[1] travis-ci/travis-ci#4596

Rakefile

@@ -23,12 +23,20 @@ end
 
 desc "verify that commit messages match CONTRIBUTING.md requirements"
 task(:commits) do
-  commits = ENV['TRAVIS_COMMIT_RANGE']
-  if commits.nil?
-    puts "TRAVIS_COMMIT_RANGE is undefined, I don't know what to check."
-    exit
-  end
-
+  # This rake task looks at the summary from every commit from this branch not
+  # in the branch targeted for a PR. This is accomplished by using the
+  # TRAVIS_COMMIT_RANGE environment variable, which is present in travis CI and
+  # populated with the range of commits the PR contains. If not available, this
+  # falls back to `main..HEAD` as a next best bet as `main` is unlikely to
+  # ever be absent.
+  #
+  # When we move to GH actions, use `GITHUB_BASE_REF` to resolve the merge base
+  # ref, which is the common ancestor between the base branch and PR. Then do
+  # git log for all of the commits in `HEAD` that are not in the base ref
+  #
+  #   baseref = %x{git merge-base HEAD $GITHUB_BASE_REF}
+  #   commits = "#{baseref}..HEAD"
+  commits = ENV['TRAVIS_COMMIT_RANGE'].nil? ? 'main..HEAD' : ENV['TRAVIS_COMMIT_RANGE'].sub(/\.\.\./, '..')
   %x{git log --no-merges --pretty=%s #{commits}}.each_line do |commit_summary|
     error_message=<<-HEREDOC
 \n\n\n\tThis commit summary didn't match CONTRIBUTING.md guidelines:\n \