Merge branch 'jk/maint-status-porcelain-z-b' into maint

"git status --porcelain" ignored "--branch" option by mistake. The output for "git status --branch -z" was also incorrect and did not terminate the record for the current branch name with NUL as asked. By Jeff King * jk/maint-status-porcelain-z-b: status: respect "-b" for porcelain format status: fix null termination with "-b" status: refactor null_termination option commit: refactor option parsing