editors/mg: Update to 20240405

ChangeLog: https://cvsweb.openbsd.org/src/usr.bin/mg/

• Reinstate space-to-tabstop entry in manpage
• Call vttidy in the exit path for batch mode
• Fix hang for (regexp-replace "^.*$" "")

PR: 278248
Reported by: cdr.nil@gmail.com