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>

trace2/tr2_sid.c

@@ -32,7 +32,7 @@ static void tr2_sid_append_my_sid_component(void)
 {
 	const struct git_hash_algo *algo = &hash_algos[GIT_HASH_SHA1];
 	struct tr2_tbuf tb_now;
-	git_hash_ctx ctx;
+	struct git_hash_ctx ctx;
 	pid_t pid = getpid();
 	unsigned char hash[GIT_MAX_RAWSZ + 1];
 	char hex[GIT_MAX_HEXSZ + 1];