sdscatfmt call sdsMakeRoomFor, asked for more space than intended (#8286)
sdscatfmt call sdsMakeRoomFor, asked for more space than intended (#8286)

instead of asking for the extra new space it wanted, it asked to grow the
string by the size it already has too.
i.e. a string of 1000 bytes, needing to grow by 10 bytes, would have been
asking for an **additional** 1010 bytes.
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