Avoid short writes in LineWriter

[ChrisDenton]

If the bytes written to LineWriter contains at least one new line but doesn't end in a new line (e.g. "abc\ndef") then we:

• write up to the last new line direct to the underlying Writer.
• copy as many of the remaining bytes as will fit into our internal buffer.

That last step is inefficient if the remaining bytes are larger than our buffer. It will needlessly split the bytes in two, requiring at least two writes to the underlying Writer (one to flush the buffer, one more to write the rest). This PR skips the extra buffering if the remaining bytes are larger than the buffer.

[rustbot]

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