hash: convert `global_init` macros to real function

The `git_hash_global_init` function is simply defined as a macro to zero
for most of the different hash implementations. This makes it impossible
to treat it like a function pointer, which is required for a later
commit where we want to improve the way global initialization works.
Fix the issue by converting all no-op macros to an inline function
returning zero.

There's a small gotcha here, though: as most hash implementations only
have a header file, but not a corresponding implementation file, we
cannot declare the function as non-static. But declaring it as `static
inline` fails, too, as there is a previous declaration as non-static. So
we have to move the function declaration after the include that brings
in the function definition, as it is allowed to have a non-static
declaration after a static definition, but not the other way round.

src/hash.h

@@ -14,7 +14,6 @@
 typedef struct git_hash_prov git_hash_prov;
 typedef struct git_hash_ctx git_hash_ctx;
 
-int git_hash_global_init(void);
 int git_hash_ctx_init(git_hash_ctx *ctx);
 void git_hash_ctx_cleanup(git_hash_ctx *ctx);
 
@@ -32,6 +31,8 @@ void git_hash_ctx_cleanup(git_hash_ctx *ctx);
 # include "hash/hash_generic.h"
 #endif
 
+int git_hash_global_init(void);
+
 typedef struct {
 	void *data;
 	size_t len;

src/hash/hash_collisiondetect.h

@@ -15,10 +15,14 @@ struct git_hash_ctx {
 	SHA1_CTX c;
 };
 
-#define git_hash_global_init() 0
 #define git_hash_ctx_init(ctx) git_hash_init(ctx)
 #define git_hash_ctx_cleanup(ctx)
 
+GIT_INLINE(int) git_hash_global_init(void)
+{
+	return 0;
+}
+
 GIT_INLINE(int) git_hash_init(git_hash_ctx *ctx)
 {
 	assert(ctx);

src/hash/hash_common_crypto.h

@@ -18,10 +18,14 @@ struct git_hash_ctx {
 
 #define CC_LONG_MAX ((CC_LONG)-1)
 
-#define git_hash_global_init() 0
 #define git_hash_ctx_init(ctx) git_hash_init(ctx)
 #define git_hash_ctx_cleanup(ctx)
 
+GIT_INLINE(int) git_hash_global_init(void)
+{
+	return 0;
+}
+
 GIT_INLINE(int) git_hash_init(git_hash_ctx *ctx)
 {
 	assert(ctx);

src/hash/hash_generic.h

@@ -18,8 +18,12 @@ struct git_hash_ctx {
 	unsigned int W[16];
 };
 
-#define git_hash_global_init() 0
 #define git_hash_ctx_init(ctx) git_hash_init(ctx)
 #define git_hash_ctx_cleanup(ctx)
 
+GIT_INLINE(int) git_hash_global_init(void)
+{
+	return 0;
+}
+
 #endif

src/hash/hash_mbedtls.h

@@ -14,7 +14,11 @@ struct git_hash_ctx {
     mbedtls_sha1_context c;
 };
 
-#define git_hash_global_init() 0
 #define git_hash_ctx_init(ctx) git_hash_init(ctx)
 
+GIT_INLINE(int) git_hash_global_init(void)
+{
+	return 0;
+}
+
 #endif /* INCLUDE_hash_mbedtld_h__ */

src/hash/hash_openssl.h

@@ -16,10 +16,14 @@ struct git_hash_ctx {
 	SHA_CTX c;
 };
 
-#define git_hash_global_init() 0
 #define git_hash_ctx_init(ctx) git_hash_init(ctx)
 #define git_hash_ctx_cleanup(ctx)
 
+GIT_INLINE(int) git_hash_global_init(void)
+{
+	return 0;
+}
+
 GIT_INLINE(int) git_hash_init(git_hash_ctx *ctx)
 {
 	assert(ctx);