finalize_object_file(): implement collision check

We've had "FIXME!!! Collision check here ?" in finalize_object_file() since aac1794132 (Improve sha1 object file writing., 2005-05-03). That is, when we try to write a file with the same name, we assume the on-disk contents are the same and blindly throw away the new copy. One of the reasons we never implemented this is because the files it moves are all named after the cryptographic hash of their contents (either loose objects, or packs which have their hash in the name these days). So we are unlikely to see such a collision by accident. And even though there are weaknesses in sha1, we assume they are mitigated by our use of sha1dc. So while it's a theoretical concern now, it hasn't been a priority. However, if we start using weaker hashes for pack checksums and names, this will become a practical concern. So in preparation, let's actually implement a byte-for-byte collision check. The new check will cause the write of new differing content to be a failure, rather than a silent noop, and we'll retain the temporary file on disk. If there's no collision present, we'll clean up the temporary file as usual after either rename()-ing or link()-ing it into place. Note that this may cause some extra computation when the files are in fact identical, but this should happen rarely. Loose objects are exempt from this check, and the collision check may be skipped by calling the _flags variant of this function with the FOF_SKIP_COLLISION_CHECK bit set. This is done for a couple of reasons: - We don't treat the hash of the loose object file's contents as a checksum, since the same loose object can be stored using different bytes on disk (e.g., when adjusting core.compression, using a different version of zlib, etc.). This is fundamentally different from cases where finalize_object_file() is operating over a file which uses the hash value as a checksum of the contents. In other words, a pair of identical loose objects can be stored using different bytes on disk, and that should not be treated as a collision. - We already use the path of the loose object as its hash value / object name, so checking for collisions at the content level doesn't add anything. Adding a content-level collision check would have to happen at a higher level than in finalize_object_file(), since (avoiding race conditions) writing an object loose which already exists in the repository will prevent us from even reaching finalize_object_file() via the object freshening code. There is a collision check in index-pack via its `check_collision()` function, but there isn't an analogous function in unpack-objects, which just feeds the result to write_object_file(). So skipping the collision check here does not change for better or worse the hardness of loose object writes. As a small note related to the latter bullet point above, we must teach the tmp-objdir routines to similarly skip the content-level collision checks when calling migrate_one() on a loose object file, which we do by setting the FOF_SKIP_COLLISION_CHECK bit when we are inside of a loose object shard. Co-authored-by: Jeff King <peff@peff.net> Signed-off-by: Jeff King <peff@peff.net> Helped-by: Elijah Newren <newren@gmail.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>