sync_default_gems.rb: ensure that commit messages end with \n

Commit messages written on GitHub's Web UI apparently use \r\n as a
line separator and do not have a \n at the end of message, unlike those
normally made with the git CLI. This breaks the expectation of the
regexps used later.

tool/sync_default_gems.rb

@@ -443,6 +443,7 @@ module SyncDefaultGems
   def message_filter(repo, sha, input: ARGF)
     log = input.read
     log.delete!("\r")
+    log << "\n" if !log.end_with?("\n")
     repo_url = "https://github.com/#{repo}"
     subject, log = log.split(/\n(?:[ \t]*(?:\n|\z))/, 2)
     conv = proc do |s|