Makefile: allow specifying a SHA-1 for non-cryptographic uses

Introduce _UNSAFE variants of the OPENSSL_SHA1, BLK_SHA1, and
APPLE_COMMON_CRYPTO_SHA1 compile-time knobs which indicate which SHA-1
implementation is to be used for non-cryptographic uses.

There are a couple of small implementation notes worth mentioning:

  - There is no way to select the collision detecting SHA-1 as the
    "fast" fallback, since the fast fallback is only for
    non-cryptographic uses, and is meant to be faster than our
    collision-detecting implementation.

  - There are no similar knobs for SHA-256, since no collision attacks
    are presently known and thus no collision-detecting implementations
    actually exist.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -521,6 +521,10 @@ include shared.mak
 # Define APPLE_COMMON_CRYPTO_SHA1 to use Apple's CommonCrypto for
 # SHA-1.
 #
+# Define the same Makefile knobs as above, but suffixed with _UNSAFE to
+# use the corresponding implementations for unsafe SHA-1 hashing for
+# non-cryptographic purposes.
+#
 # If don't enable any of the *_SHA1 settings in this section, Git will
 # default to its built-in sha1collisiondetection library, which is a
 # collision-detecting sha1 This is slower, but may detect attempted
@@ -1987,6 +1991,27 @@ endif
 endif
 endif
 
+ifdef OPENSSL_SHA1_UNSAFE
+ifndef OPENSSL_SHA1
+	EXTLIBS += $(LIB_4_CRYPTO)
+	BASIC_CFLAGS += -DSHA1_OPENSSL_UNSAFE
+endif
+else
+ifdef BLK_SHA1_UNSAFE
+ifndef BLK_SHA1
+	LIB_OBJS += block-sha1/sha1.o
+	BASIC_CFLAGS += -DSHA1_BLK_UNSAFE
+endif
+else
+ifdef APPLE_COMMON_CRYPTO_SHA1_UNSAFE
+ifndef APPLE_COMMON_CRYPTO_SHA1
+	COMPAT_CFLAGS += -DCOMMON_DIGEST_FOR_OPENSSL
+	BASIC_CFLAGS += -DSHA1_APPLE_UNSAFE
+endif
+endif
+endif
+endif
+
 ifdef OPENSSL_SHA256
 	EXTLIBS += $(LIB_4_CRYPTO)
 	BASIC_CFLAGS += -DSHA256_OPENSSL

hash.h

@@ -15,6 +15,36 @@
 #include "block-sha1/sha1.h"
 #endif
 
+#if defined(SHA1_APPLE_UNSAFE)
+#  include <CommonCrypto/CommonDigest.h>
+#  define platform_SHA_CTX_unsafe CC_SHA1_CTX
+#  define platform_SHA1_Init_unsafe CC_SHA1_Init
+#  define platform_SHA1_Update_unsafe CC_SHA1_Update
+#  define platform_SHA1_Final_unsafe CC_SHA1_Final
+#elif defined(SHA1_OPENSSL_UNSAFE)
+#  include <openssl/sha.h>
+#  if defined(OPENSSL_API_LEVEL) && OPENSSL_API_LEVEL >= 3
+#    define SHA1_NEEDS_CLONE_HELPER_UNSAFE
+#    include "sha1/openssl.h"
+#    define platform_SHA_CTX_unsafe openssl_SHA1_CTX
+#    define platform_SHA1_Init_unsafe openssl_SHA1_Init
+#    define platform_SHA1_Clone_unsafe openssl_SHA1_Clone
+#    define platform_SHA1_Update_unsafe openssl_SHA1_Update
+#    define platform_SHA1_Final_unsafe openssl_SHA1_Final
+#  else
+#    define platform_SHA_CTX_unsafe SHA_CTX
+#    define platform_SHA1_Init_unsafe SHA1_Init
+#    define platform_SHA1_Update_unsafe SHA1_Update
+#    define platform_SHA1_Final_unsafe SHA1_Final
+#  endif
+#elif defined(SHA1_BLK_UNSAFE)
+#  include "block-sha1/sha1.h"
+#  define platform_SHA_CTX_unsafe blk_SHA_CTX
+#  define platform_SHA1_Init_unsafe blk_SHA1_Init
+#  define platform_SHA1_Update_unsafe blk_SHA1_Update
+#  define platform_SHA1_Final_unsafe blk_SHA1_Final
+#endif
+
 #if defined(SHA256_NETTLE)
 #include "sha256/nettle.h"
 #elif defined(SHA256_GCRYPT)