When fsync'ing files, fsync the parent directory in the case where we rename a file into place, or create a new file, to ensure that the directory entry is flushed correctly.
Add a custom `O_FSYNC` bit (if it's not been defined by the operating system`) so that `git_futils_writebuffer` can optionally do an `fsync` when it's done writing. We call `fsync` ourselves, even on systems that define `O_FSYNC` because its definition is no guarantee of its actual support. Mac, for instance, defines it but doesn't support it in an `open(2)` call.
Since writing multiple objects may all already exist in a single packfile, avoid freshening that packfile repeatedly in a tight loop. Instead, only freshen pack files every 2 seconds.
When writing an object, we calculate its OID and see if it exists in the object database. If it does, we need to freshen the file that contains it.
git_index_entry__init_from_stat: set nsec fields in entry stats
Instead of relying on the size and timestamp, which can hide changes performed in the same second, hash the file content's when we care about detecting changes.
Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter assumes that we own everything beneath the base, as if it were being called with a base of the repository or working directory, and is tailored towards checkout and ensuring that there is no bogosity beneath the base that must be cleaned up. This is (at best) slow and (at worst) unsafe in the larger context of a filesystem where we do not own things and cannot do things like unlink symlinks that are in our way.