reftable/block: open-code call to `uncompress2()`

The reftable format stores log blocks in a compressed format. Thus, whenever we want to read such a block we first need to decompress it. This is done by calling the convenience function `uncompress2()` of the zlib library, which is a simple wrapper that manages the lifecycle of the `zstream` structure for us. While nice for one-off inflation of data, when iterating through reflogs we will likely end up inflating many such log blocks. This requires us to reallocate the state of the `zstream` every single time, which adds up over time. It would thus be great to reuse the `zstream` instead of discarding it after every inflation. Open-code the call to `uncompress2()` such that we can start reusing the `zstream` in the subsequent commit. Note that our open-coded variant is different from `uncompress2()` in two ways: - We do not loop around `inflate()` until we have processed all input. As our input is limited by the maximum block size, which is 16MB, we should not hit limits of `inflate()`. - We use `Z_FINISH` instead of `Z_NO_FLUSH`. Quoting the `inflate()` documentation: "inflate() should normally be called until it returns Z_STREAM_END or an error. However if all decompression is to be performed in a single step (a single call of inflate), the parameter flush should be set to Z_FINISH." Furthermore, "Z_FINISH also informs inflate to not maintain a sliding window if the stream completes, which reduces inflate's memory footprint." Other than that this commit is expected to be functionally equivalent and does not yet reuse the `zstream`. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>