remotefilelog: replace repack lock to solve race condition 2c74337e6483 reduced the probability of race-conditions when starting background repack and prefetch and we saw the difference in our CI instance with all failures disappearing except one where one call to waitonrepack seems to returns too early. I'm not sure what exactly goes wrong but I realized that while the prefetch operation uses a standard Mercurial lock, the repack operation is using a custom lock based on `fcntl.flock` on available platforms. As `extutil.flock` fallback on traditional Mercurial locks on other platforms and the tests are stable on my laptop, our CI environment and GCC112, I'm sending this patch to standardize the behavior across environments. Differential Revision: https://phab.mercurial-scm.org/D6844