hash: add discard primitive

The usual life-cycle for a git_hash_ctx is calling git_hash_init(), adding some data, and then using git_hash_final() to get the output digest and free any resources. Sometimes we decide to abort the operation without the final() call (e.g., due to errors or other reasons). In that case we just abandon the hash_ctx completely and let it go out of scope. For most hash implementations this is fine; they were just holding values directly in the struct. But some implementations do allocate memory, and in these cases we leak the memory. Notably OpenSSL >= 3.0 requires us to allocate the digest context on the heap with EVP_MD_CTX_new(). Let's provide a git_hash_discard() function that can be used in these code paths to free any resources. For now we'll implement it by just calling git_hash_final() into a dummy output, relying on its side effect of freeing the resources. Our view of the underlying hash implementation is abstracted behind the platform_SHA_* macros, so that's the best we can do without widening that interface. It's a little inefficient, but probably not noticeably so in practice, especially as we'd usually hit this on an error code path. And by abstracting it in this function, we can later swap it out when the platform_SHA interface lets us do so. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>