hash: stop typedeffing the hash context

We generally avoid using `typedef` in the Git codebase. One exception
though is the `git_hash_ctx`, likely because it used to be a union
rather than a struct until the preceding commit refactored it. But now
that it is a normal `struct` there isn't really a need for a typedef
anymore.

Drop the typedef and adapt all callers accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/u-hash.c

@@ -8,7 +8,7 @@ static void check_hash_data(const void *data, size_t data_length,
 	cl_assert(data != NULL);
 
 	for (size_t i = 1; i < ARRAY_SIZE(hash_algos); i++) {
-		git_hash_ctx ctx;
+		struct git_hash_ctx ctx;
 		unsigned char hash[GIT_MAX_HEXSZ];
 		const struct git_hash_algo *algop = &hash_algos[i];