remote.c: untangle error logic in branch_get_upstream

The error-diagnosis logic in branch_get_upstream was copied
straight from sha1_name.c in the previous commit. However,
because we check all error cases and upfront and then later
diagnose them, the logic is a bit tangled. In particular:

  - if branch->merge[0] is NULL, we may end up dereferencing
    it for an error message (in practice, it should never be
    NULL, so this is probably not a triggerable bug).

  - We may enter the code path because branch->merge[0]->dst
    is NULL, but we then start our error diagnosis by
    checking whether our local branch exists. But that is
    only relevant to diagnosing missing merge config, not a
    missing tracking ref; our diagnosis may hide the real
    problem.

Instead, let's just use a sequence of "if" blocks to check
for each error type, diagnose it, and return NULL.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -1721,18 +1721,25 @@ const char *branch_get_upstream(struct branch *branch, struct strbuf *err)
 {
 	if (!branch)
 		return error_buf(err, _("HEAD does not point to a branch"));
-	if (!branch->merge || !branch->merge[0] || !branch->merge[0]->dst) {
+
+	if (!branch->merge || !branch->merge[0]) {
+		/*
+		 * no merge config; is it because the user didn't define any,
+		 * or because it is not a real branch, and get_branch
+		 * auto-vivified it?
+		 */
 		if (!ref_exists(branch->refname))
 			return error_buf(err, _("no such branch: '%s'"),
 					 branch->name);
-		if (!branch->merge)
-			return error_buf(err,
-					 _("no upstream configured for branch '%s'"),
-					 branch->name);
+		return error_buf(err,
+				 _("no upstream configured for branch '%s'"),
+				 branch->name);
+	}
+
+	if (!branch->merge[0]->dst)
 		return error_buf(err,
 				 _("upstream branch '%s' not stored as a remote-tracking branch"),
 				 branch->merge[0]->src);
-	}
 
 	return branch->merge[0]->dst;
 }