sha1: provide another level of indirection for the SHA-1 functions

The git source uses git_SHA1_Update() and friends to call into the
code that computes the hashes.  Traditionally, we used to map these
directly to underlying implementation of the SHA-1 hash (e.g.
SHA1_Update() from OpenSSL or blk_SHA1_Update() from block-sha1/).

This arrangement however makes it hard to tweak behaviour of the
underlying implementation without fully replacing.  If we want to
introduce a tweaked_SHA1_Update() wrapper to implement the "Update"
in a slightly different way, for example, the implementation of the
wrapper still would want to call into the underlying implementation,
but tweaked_SHA1_Update() cannot call git_SHA1_Update() to get to
the underlying implementation (often but not always SHA1_Update()).

Add another level of indirection that maps platform_SHA1_Update()
and friends to their underlying implementations, and by default make
git_SHA1_Update() and friends map to platform_SHA1_* functions.

Doing it this way will later allow us to map git_SHA1_Update() to
tweaked_SHA1_Update(), and the latter can use platform_SHA1_Update()
in its implementation.

Signed-off-by: Atousa Pahlevan Duprat <apahlevan@ieee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

block-sha1/sha1.h

@@ -16,7 +16,7 @@ void blk_SHA1_Init(blk_SHA_CTX *ctx);
 void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *dataIn, unsigned long len);
 void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx);
 
-#define git_SHA_CTX	blk_SHA_CTX
-#define git_SHA1_Init	blk_SHA1_Init
-#define git_SHA1_Update	blk_SHA1_Update
-#define git_SHA1_Final	blk_SHA1_Final
+#define platform_SHA_CTX	blk_SHA_CTX
+#define platform_SHA1_Init	blk_SHA1_Init
+#define platform_SHA1_Update	blk_SHA1_Update
+#define platform_SHA1_Final	blk_SHA1_Final

cache.h

@@ -11,13 +11,25 @@
 #include "string-list.h"
 
 #include SHA1_HEADER
-#ifndef git_SHA_CTX
-#define git_SHA_CTX	SHA_CTX
-#define git_SHA1_Init	SHA1_Init
-#define git_SHA1_Update	SHA1_Update
-#define git_SHA1_Final	SHA1_Final
+#ifndef platform_SHA_CTX
+/*
+ * platform's underlying implementation of SHA-1; could be OpenSSL,
+ * blk_SHA, Apple CommonCrypto, etc...  Note that including
+ * SHA1_HEADER may have already defined platform_SHA_CTX for our
+ * own implementations like block-sha1 and ppc-sha1, so we list
+ * the default for OpenSSL compatible SHA-1 implementations here.
+ */
+#define platform_SHA_CTX	SHA_CTX
+#define platform_SHA1_Init	SHA1_Init
+#define platform_SHA1_Update	SHA1_Update
+#define platform_SHA1_Final    	SHA1_Final
 #endif
 
+#define git_SHA_CTX		platform_SHA_CTX
+#define git_SHA1_Init		platform_SHA1_Init
+#define git_SHA1_Update		platform_SHA1_Update
+#define git_SHA1_Final		platform_SHA1_Final
+
 #include <zlib.h>
 typedef struct git_zstream {
 	z_stream z;

ppc/sha1.h

@@ -19,7 +19,7 @@ int ppc_SHA1_Init(ppc_SHA_CTX *c);
 int ppc_SHA1_Update(ppc_SHA_CTX *c, const void *p, unsigned long n);
 int ppc_SHA1_Final(unsigned char *hash, ppc_SHA_CTX *c);
 
-#define git_SHA_CTX	ppc_SHA_CTX
-#define git_SHA1_Init	ppc_SHA1_Init
-#define git_SHA1_Update	ppc_SHA1_Update
-#define git_SHA1_Final	ppc_SHA1_Final
+#define platform_SHA_CTX	ppc_SHA_CTX
+#define platform_SHA1_Init	ppc_SHA1_Init
+#define platform_SHA1_Update	ppc_SHA1_Update
+#define platform_SHA1_Final	ppc_SHA1_Final