use the latest HEAD/master of the official SwiftLint repo as 'master'

otherwise, if forks have an out-of-date `master` branch, OSSCheck will be
wildly inaccurate.

script/oss-check

@@ -236,6 +236,10 @@ def diff_and_report_changes_to_danger
   end
 end
 
+def fetch_origin
+  `git fetch origin`
+end
+
 def clean_up
   FileUtils.rm_rf(@working_dir)
   perform('git worktree prune')
@@ -267,8 +271,9 @@ $stdout.sync = true
 validate_state_to_run
 setup_repos
 make_directory_structure
+fetch_origin
 
-@effective_master_commitish = `git merge-base master #{@options[:branch]}`
+@effective_master_commitish = `git merge-base origin/master #{@options[:branch]}`
 
 # Build & generate reports for branch & master
 %w[branch master].each do |branch|