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.

routers/web/repo/pull.go

@@ -1308,7 +1308,7 @@ func PullsNewRedirect(ctx *context.Context) {
 			return
 		}
 		redirectRepo = repo.BaseRepo
-		branch = fmt.Sprintf("%s:%s", repo.OwnerName, branch)
+		branch = context.CompareHeadRef(repo, branch)
 	}
 	ctx.Redirect(fmt.Sprintf("%s/compare/%s...%s?expand=1", redirectRepo.Link(), util.PathEscapeSegments(redirectRepo.DefaultBranch), util.PathEscapeSegments(branch)))
 }

services/context/repo.go

@@ -64,11 +64,15 @@ func (prc *PullRequestContext) CanCreateNewPull() bool {
 	return can
 }
 
+// CompareHeadRef formats the head side of a compare link, "owner/repo:branch" is only needed when a fork can share its base repo's owner
+func CompareHeadRef(headRepo *repo_model.Repository, headBranch string) string {
+	return util.Iif(setting.Repository.AllowForkIntoSameOwner, headRepo.FullName(), headRepo.OwnerName) + ":" + headBranch
+}
+
 func (prc *PullRequestContext) MakeDefaultCompareLink(headBranch string) string {
 	return prc.baseRepo.Link() + "/compare/" +
 		util.PathEscapeSegments(prc.DefaultTargetBranch()) + "..." +
-		util.Iif(prc.SameRepo(), "", util.PathEscapeSegments(prc.headRepo.OwnerName)+":") +
-		util.PathEscapeSegments(headBranch)
+		util.PathEscapeSegments(util.Iif(prc.SameRepo(), headBranch, CompareHeadRef(prc.headRepo, headBranch)))
 }
 
 func (prc *PullRequestContext) DefaultTargetBranch() string {

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"))
+}