Merge branch 'pw/rebase-x-sanity-check'

"git rebase -x $cmd" did not reject multi-line command, even though
the command is incapable of handling such a command.  It now is
rejected upfront.

* pw/rebase-x-sanity-check:
  rebase -x: sanity check command

diff --cc builtin/rebase.c
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge