remote: use git_branch_upstream_remote()

This reduces the clutter somewhat and lets us see what we're asking
about the reference.

src/remote.c

@@ -936,9 +936,10 @@ static int remote_head_for_ref(git_remote_head **out, git_remote *remote, git_re
 	git_buf remote_name = GIT_BUF_INIT;
 	git_buf upstream_name = GIT_BUF_INIT;
 	git_buf config_key = GIT_BUF_INIT;
+	git_buf upstream_remote = GIT_BUF_INIT;
 	git_repository *repo;
 	git_config *config = NULL;
-	const char *ref_name, *branch_remote;
+	const char *ref_name;
 	int error = 0;
 
 	assert(out && spec && ref);
@@ -958,17 +959,16 @@ static int remote_head_for_ref(git_remote_head **out, git_remote *remote, git_re
 	}
 
 	if ((!git_reference__is_branch(ref_name)) ||
-	    (error = git_repository_config_snapshot(&config, repo)) < 0 ||
-	    (error = git_buf_printf(&config_key, "branch.%s.remote",
-	        ref_name + strlen(GIT_REFS_HEADS_DIR))) < 0 ||
-	    (error = git_config_get_string(&branch_remote, config, git_buf_cstr(&config_key))) < 0 ||
-	    git__strcmp(git_remote_name(remote), branch_remote) ||
+	    (error = git_branch_upstream_remote(&upstream_remote, repo, ref_name)) ||
+	    git__strcmp(git_remote_name(remote), git_buf_cstr(&upstream_remote)) ||
 	    (error = git_branch_upstream_name(&upstream_name, repo, ref_name)) < 0 ||
 	    !git_refspec_dst_matches(spec, git_buf_cstr(&upstream_name)) ||
 	    (error = git_refspec_rtransform(&remote_name, spec, upstream_name.ptr)) < 0) {
 		/* Not an error if there is no upstream */
-		if (error == GIT_ENOTFOUND)
+		if (error == GIT_ENOTFOUND) {
+			giterr_clear();
 			error = 0;
+		}
 
 		goto cleanup;
 	}
@@ -977,6 +977,7 @@ static int remote_head_for_ref(git_remote_head **out, git_remote *remote, git_re
 
 cleanup:
 	git_reference_free(resolved_ref);
+	git_buf_free(&upstream_remote);
 	git_buf_free(&remote_name);
 	git_buf_free(&upstream_name);
 	git_buf_free(&config_key);