hash: add an SHA-256 implementation using OpenSSL

We already have OpenSSL routines available for SHA-1, so add routines
for SHA-256 as well.

On a Core i7-6600U, this SHA-256 implementation compares favorably to
the SHA1DC SHA-1 implementation:

SHA-1: 157 MiB/s (64 byte chunks); 337 MiB/s (16 KiB chunks)
SHA-256: 165 MiB/s (64 byte chunks); 408 MiB/s (16 KiB chunks)

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -183,6 +183,8 @@ all::
 #
 # Define GCRYPT_SHA256 to use the SHA-256 routines in libgcrypt.
 #
+# Define OPENSSL_SHA256 to use the SHA-256 routines in OpenSSL.
+#
 # Define NEEDS_CRYPTO_WITH_SSL if you need -lcrypto when using -lssl (Darwin).
 #
 # Define NEEDS_SSL_WITH_CRYPTO if you need -lssl when using -lcrypto (Darwin).
@@ -1638,13 +1640,18 @@ endif
 endif
 endif
 
+ifdef OPENSSL_SHA256
+	EXTLIBS += $(LIB_4_CRYPTO)
+	BASIC_CFLAGS += -DSHA256_OPENSSL
+else
 ifdef GCRYPT_SHA256
 	BASIC_CFLAGS += -DSHA256_GCRYPT
 	EXTLIBS += -lgcrypt
 else
 	LIB_OBJS += sha256/block/sha256.o
 	BASIC_CFLAGS += -DSHA256_BLK
 endif
+endif
 
 ifdef SHA1_MAX_BLOCK_SIZE
 	LIB_OBJS += compat/sha1-chunked.o

hash.h

@@ -17,6 +17,8 @@
 
 #if defined(SHA256_GCRYPT)
 #include "sha256/gcrypt.h"
+#elif defined(SHA256_OPENSSL)
+#include <openssl/sha.h>
 #else
 #include "sha256/block/sha256.h"
 #endif