Backport #16517
The CompareAndPullRequestPost handler for POST to /compare
incorrectly handles returning errors to the user. For a start
it does not set the necessary markers to switch SimpleMDE
but it also does not immediately return to the form.
This PR fixes this by setting the appropriate values, fixing
the templates and preventing the suggestion of a too long
title.
Fix#16507
Signed-off-by: Andrew Thornton <art27@cantab.net>
