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>

http.h

@@ -228,7 +228,7 @@ struct http_object_request {
 	long http_code;
 	struct object_id oid;
 	struct object_id real_oid;
-	git_hash_ctx c;
+	struct git_hash_ctx c;
 	git_zstream stream;
 	int zret;
 	int rename;