• fix vi mode (which, however, is officially orphaned) multi-line $PS1 by
using a similar algorithm for prompt skipping as emacs mode (changing
the meaning of prompt_trunc variable and using prompt_redraw, just even
more efficiently than vi mode); reported by asarch via IRC
• fix multi-line prompts if last line is “too large” by using emacs mode
algorithm of just internally appending a newline, while here ☺ this even
saves us having to re-add the prompt_skip variable…
WARNING: this is only barely tested, as almost nobody ever uses vi mode
⇒ test yourself, there may be bugs (e.g. off-by-ones); already known is
that the vi input line editing mode is NOT multibyte safe…
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