hash: add discard primitive

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>