fastexport: rework newline logic Newlines should only be added when otherwise the stream would look weird without them (on blobs), therefore they are the exception. Flip the logic so they are added, not skipped.

encoding: avoid quadratic time complexity when json-encoding non-UTF8 strings Apparently the code uses "+=" with a bytes object, which is linear-time, so the whole encoding is quadratic-time. This patch makes us use a bytearray object, instead, which has a(n amortized-)constant-time append operation. The encoding is still not particularly fast, but at least a 10MB file takes tens of seconds, not many hours to encode.