Add new line when rewriting git message

Update the forward merge script to add an additional new line when
rewiring the commit message. Prior this this commit, if an additional
line immediately followed the "Fixes ... in ..." pattern it would be
appended to the end of the rewritten fixes line.

Closes gh-16861

Author: philwebb

git/hooks/forward-merge

@@ -84,5 +84,5 @@ password = config['github']['credentials']['password']
 dry_run = config['dry_run']
 new_issue_number = create_issue(username, password, repository, forward_merge.issue, title, labels, milestone, forward_merge.milestone, dry_run)
 puts "Created gh-#{new_issue_number} for forward port of gh-#{forward_merge.issue} into #{forward_merge.milestone}"
-rewritten_message = forward_merge.message.sub(forward_merge.line, "Closes gh-#{new_issue_number}")
+rewritten_message = forward_merge.message.sub(forward_merge.line, "Closes gh-#{new_issue_number}\n")
 File.write(message_file, rewritten_message)