Merge branch 'rt/rebase-i-broken-insn-advise' into maint

When "git rebase -i" is given a broken instruction, it told the
user to fix it with "--edit-todo", but didn't say what the step
after that was (i.e. "--continue").

* rt/rebase-i-broken-insn-advise:
rebase -i: improve advice on bad instruction lines