hash: check ctx->active flag in all wrapper functions

hash: check ctx->active flag in all wrapper functions

It only makes sense to call git_hash_update(), etc, on a hash context
that has been initialized but not yet finalized or discarded. This is an
unlikely error to make, but it's easy for us to catch it and complain.

It's especially important because it would quietly "work" for many hash
backends (like sha1dc, which is just manipulating some bytes) but would
cause undefined behavior with others (like OpenSSL, which puts the
context onto the heap). Checking the flag lets us catch problems
consistently on every build.

Note that we can't do the same for git_hash_init(). Even though it would
cause a leak to call it twice (without an intervening final/discard),
the point of the function is that the contents of the struct are
undefined before the call. But calling it twice is an even less likely
error to make, so not covering it is OK.

We leave git_hash_discard() alone, as its idempotent behavior is
convenient for callers. We _could_ try to do something similar for
git_hash_final(), allowing:

  git_hash_final(result, &ctx);
  git_hash_final(other_result, &ctx);

but it does not make much sense. After the first final() call we have
thrown away the state, so we cannot produce the same output. We could
come up with some sensible output (the null hash, or the empty hash),
but double-calls like this are more likely a bug, so our best bet is to
complain loudly (whereas the current code produces either nonsense
output or undefined behavior, depending on the backend).

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