"git rebase -i" fired post-rewrite hook when it shouldn't (namely,
when it was told to stop sequencing with 'exec' insn).

* mm/rebase-i-post-rewrite-exec:
  t5407: use <<- to align the expected output
  rebase -i: fix post-rewrite hook with failed exec command
  rebase -i: demonstrate incorrect behavior of post-rewrite