patch: use a short, fixed-size message for last line of prompt (issue6158) See issue6158 and the previous commit for examples of what might go wrong if we have some combinations of readline version and terminal and need to wrap the line. Briefly: readline may not display the beginning of the last line of the prompt, or it may print over it with the end of the prompt, making it difficult for users to know what's going on. Differential Revision: https://phab.mercurial-scm.org/D6563