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

"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
via Jeff King
* jk/status-porcelain-z-b:
  status: refactor colopts handling
  status: respect "-b" for porcelain format
  status: fix null termination with "-b"
  status: refactor null_termination option
  commit: refactor option parsing