Remove amortized growth in `SharedBuffer::Reset()`. Resetting does not involve copying existing contents, so even if the size grows by small amounts, this does not lead to O(N²) cost.
Remove amortized growth in `SharedBuffer::Reset()`. Resetting does not involve
copying existing contents, so even if the size grows by small amounts, this does
not lead to O(N²) cost.

`Buffer::Reset()` does not use amortized growth.

PiperOrigin-RevId: 415246255

google/riegeli,google/riegeli,google/riegeli,google/riegeli