hash: make git_hash_discard() idempotent

hash: make git_hash_discard() idempotent

You must always either finalize or discard a hash context to release any
resources, but you must call only one such function. This creates extra
work for some callers, since their cleanup code paths need to know
whether they got there via their happy path (and the finalization
happened) or due to an error (in which case they need to discard).

Let's add an "active" flag that turns a redundant discard into a noop.
That lets you safely do this:

    git_hash_init(&ctx, algo);
    ...
    if (some_error)
            goto out;
    ...
    git_hash_final(result, &ctx);

  out:
    git_hash_discard(&ctx);

This should avoid future errors, and will also let us simplify a few
existing callers (in future patches).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>