fix(pull): name the head repository in default compare links (#39075)

The "New Pull Request" buttons and the `/pulls/new/{branch}` redirect
build their compare link as `{owner}:{branch}`. If a fork and its parent
share an owner, through ALLOW_FORK_INTO_SAME_OWNER, or after a transfer,
that head resolves back to the base repo, so the link compares the base
against itself and 404s on a branch that only exists in the fork.

Switching to `{owner}/{repo}:{branch}` names the head repo
unambiguously, and it's what the compare page's own links already use.

Also clears the 404 in #37649; the archived-parent half of that report
is separate.

services/context/repo_test.go

@@ -0,0 +1,25 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package context
+
+import (
+	"testing"
+
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/modules/setting"
+	"gitea.dev/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCompareHeadRef(t *testing.T) {
+	defer test.MockVariableValue(&setting.Repository.AllowForkIntoSameOwner)()
+	headRepo := &repo_model.Repository{OwnerName: "user", Name: "fork"}
+
+	setting.Repository.AllowForkIntoSameOwner = false
+	assert.Equal(t, "user:my-branch", CompareHeadRef(headRepo, "my-branch"))
+
+	setting.Repository.AllowForkIntoSameOwner = true
+	assert.Equal(t, "user/fork:my-branch", CompareHeadRef(headRepo, "my-branch"))
+}