osutil: use getdirentriesattr on OS X if possible This is a significant win for large repositories on OS X, especially with a cold cache. Unfortunately we need to keep the lstat-based implementation around for two reasons: - Not all filesystems support this call. - There's an edge case in which it's best to fall back to avoid a retry loop. More about this in the comments. The below tests are all performed on a Mac with an SSD running OS X 10.9, on a repository with over 200k files. The results are best of 5 with simulated best-effort conditions. The gains with a hot cache are pretty impressive: 'hg status' goes from 5.18 seconds to 3.79 seconds. However, a repository that large will probably already be using something like hgwatchman [1], which helps much more (for this repo, 'hg status' with hgwatchman is approximately 1 second). Where this really helps is when the cache is cold [2]: hg status goes from 31.0 seconds to 9.66. See http://lists.apple.com/archives/filesystem-dev/2014/Dec/msg00002.html for some more discussion about this function. This is based on a patch by Sean Farley <sean@farley.io>. [1] https://bitbucket.org/facebook/hgwatchman [2] There appears to be no easy way to clear the file cache (aka "vnodes") on OS X short of rebooting. purge(8) purportedly does that but in my testing had little effect. The workaround I came up with was to assume that vnode eviction was LRU, make sure the kern.maxvnodes sysctl is smaller than the size of the repository, then make sure we'd always miss the cache by running 'hg status' in another clone of the repository before running it in the test repository.