improve error message for 'hg merge' when repo already at branchtip
adding helloabort: '\n' and '\r' disallowed in filenamesadding helloabort: '\n' and '\r' disallowed in filenamesadding helloabort: '\n' and '\r' disallowed in filenamesadding helloabort: '\n' and '\r' disallowed in filenamesf hello hellof hello hello