patchbomb: make it easy for the user to decline sending an intro message.
- prompt(): respect interactive mode; clarify logic a bit
- rename introneeded() to introwanted() and give it only one caller
- add 'numbered' arg to makepatch() so it does not need to call
introwanted()
- factor makeintro() out of getpatchmsgs(), so it's easier to skip the
intro message based on the user's behaviour
Unexpected but perfectly reasonable side effect: in non-interactive
mode, we don't show unanswerable "Cc" or "From" prompts anymore, so
remove those from the test expectations.
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