csum-file: always finalize or discard hash

When a hashfile struct is created, we always initialize the git_hash_ctx
inside it. We usually end up in hashfile_finalize(), which passes that
ctx to git_hash_final(), cleaning it up.

But a few code paths don't do so:

  1. If we bail on the hashfile and call free_hashfile() directly rather
     than finalizing.

  2. If the skip_hash flag is set, the hashfile_finalize() call will
     never call git_hash_final(). (You might think that we should just
     avoid git_hash_init() entirely in this case, but the skip_hash flag
     is set by the caller after the hashfile is initialized).

For most hash implementations this is OK, but for ones that allocate on
initialization it causes a memory leak. You can see many failures by
running:

  make SANITIZE=leak OPENSSL_SHA1_UNSAFE=1 test

since OpenSSL >= 3.0 is such an allocating hash implementation (and
csum-file uses the "unsafe" algorithm variant).

We can solve this by calling git_hash_discard() as appropriate.

Note that free_hashfile() is used both directly by callers to abort
without finalizing, and by hashfile_finalize() to free memory. In the
latter case we _don't_ want to call git_hash_discard(), because we'll
already have either finalized or discarded it. So we'll push that to an
internal "free_memory" function, and keep free_hashfile() as the public
interface to abort a hashfile without finalizing.

This fix makes several scripts leak-free with the command above: t1600,
t1601, t2107, t7008, t9210, t9211.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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