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

diff --cc Documentation/git-status.txt
Simple merge