JLine 3: `:paste` is misbehaving (continuation prompt should be suppressed)

[SethTisue]

reported by @bishabosha at scala/scala#8036 (comment)

[SethTisue]

I had initially thought the old continuation-handling code must be interfering, but no, it's simpler than that.

in Reader.scala, we do:

        .variable(SECONDARY_PROMPT_PATTERN, config.encolor(config.continueText))

and this then applies to regular mode (good) but also to paste mode (bad). so that's the cause

to fix it, in ILoop#pasteCommand we need to setVariable(LineReader.SECONDARY_PROMPT_PATTERN, "") and then restore it when we're done. but ILoop doesn't have direct access to its org.jline.reader.LineReader, so I think we need to add a method to nsc.interpreter.jline.Reader that handles this (it can be a no-op when we're running without JLine involved, as during testing)

I verified that this does the trick if done unsafely using in.asInstanceOf[jline.Reader], now I just need to do that the right way

[SethTisue]

closing on the presumption my fix will be merged