sha256/gcrypt: fix memory leak with SHA-256 repos

`gcry_md_open' needs to be paired with `gcry_md_close' to ensure
resources are released.  Since our internal APIs don't have
separate close/release callbacks, sticking it into the finalization
callback seems appropriate.

Building with SANITIZE=leak and running `git fsck' on a SHA-256
repository no longer reports leaks.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha256/gcrypt.h

@@ -20,6 +20,7 @@ static inline void gcrypt_SHA256_Update(gcrypt_SHA256_CTX *ctx, const void *data
 static inline void gcrypt_SHA256_Final(unsigned char *digest, gcrypt_SHA256_CTX *ctx)
 {
  memcpy(digest, gcry_md_read(*ctx, GCRY_MD_SHA256), SHA256_DIGEST_SIZE);
+ gcry_md_close(*ctx);
 }
 
 static inline void gcrypt_SHA256_Clone(gcrypt_SHA256_CTX *dst, const gcrypt_SHA256_CTX *src)