hash: add ability to distinguish algorithms

Create an enum that allows us to distinguish between different
hashing algorithms. This enum is embedded into each
`git_hash_ctx` and will instruct the code to which hashing
function the particular request shall be dispatched.

As we do not yet have multiple hashing algorithms, we simply
initialize the hash algorithm to always be SHA1. At a later
point, we will have to extend the `git_hash_init_ctx` function to
get as parameter which algorithm shall be used.

src/hash.c

@@ -14,27 +14,55 @@ int git_hash_global_init(void)
 
 int git_hash_ctx_init(git_hash_ctx *ctx)
 {
-	return git_hash_sha1_ctx_init(&ctx->sha1);
+	int error;
+
+	if ((error = git_hash_sha1_ctx_init(&ctx->sha1)) < 0)
+		return error;
+
+	ctx->algo = GIT_HASH_ALGO_SHA1;
+
+	return 0;
 }
 
 void git_hash_ctx_cleanup(git_hash_ctx *ctx)
 {
+	switch (ctx->algo) {
+		case GIT_HASH_ALGO_SHA1:
 			git_hash_sha1_ctx_cleanup(&ctx->sha1);
+			return;
+		default:
+			assert(0);
+	}
 }
 
 int git_hash_init(git_hash_ctx *ctx)
 {
+	switch (ctx->algo) {
+		case GIT_HASH_ALGO_SHA1:
 			return git_hash_sha1_init(&ctx->sha1);
+		default:
+			assert(0);
+	}
 }
 
 int git_hash_update(git_hash_ctx *ctx, const void *data, size_t len)
 {
+	switch (ctx->algo) {
+		case GIT_HASH_ALGO_SHA1:
 			return git_hash_sha1_update(&ctx->sha1, data, len);
+		default:
+			assert(0);
+	}
 }
 
 int git_hash_final(git_oid *out, git_hash_ctx *ctx)
 {
+	switch (ctx->algo) {
+		case GIT_HASH_ALGO_SHA1:
 			return git_hash_sha1_final(out, &ctx->sha1);
+		default:
+			assert(0);
+	}
 }
 
 int git_hash_buf(git_oid *out, const void *data, size_t len)

src/hash.h

@@ -17,12 +17,18 @@ typedef struct {
 	size_t len;
 } git_buf_vec;
 
+typedef enum {
+	GIT_HASH_ALGO_UNKNOWN = 0,
+	GIT_HASH_ALGO_SHA1,
+} git_hash_algo_t;
+
 #include "hash/sha1.h"
 
 typedef struct git_hash_ctx {
 	union {
 		git_hash_sha1_ctx sha1;
 	};
+	git_hash_algo_t algo;
 } git_hash_ctx;
 
 int git_hash_global_init(void);