Allow configuring default PR base branch (fixes #36412) (#36425)

This adds a per-repository default PR base branch and wires it through
PR entry points. It updates compare links and recently pushed branch
prompts to respect the configured base branch, and prevents auto-merge
cleanup from deleting the configured base branch on same-repo PRs.

## Behavior changes
- New PR compare links on repo home/issue list and branch list honor the
configured default PR base branch.
- The "recently pushed new branches" prompt now compares against the
configured base branch.
- Auto-merge branch cleanup skips deleting the configured base branch
(same-repo PRs only).

---------

Signed-off-by: Louis <116039387+tototomate123@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

models/repo/pull_request_default.go

@@ -0,0 +1,16 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/models/unit"
+	"code.gitea.io/gitea/modules/util"
+)
+
+func (repo *Repository) GetPullRequestTargetBranch(ctx context.Context) string {
+	unitPRConfig := repo.MustGetUnit(ctx, unit.TypePullRequests).PullRequestsConfig()
+	return util.IfZero(unitPRConfig.DefaultTargetBranch, repo.DefaultBranch)
+}

models/repo/pull_request_default_test.go

@@ -0,0 +1,32 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/unit"
+	"code.gitea.io/gitea/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDefaultTargetBranchSelection(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	ctx := t.Context()
+	repo := unittest.AssertExistsAndLoadBean(t, &Repository{ID: 1})
+
+	assert.Equal(t, repo.DefaultBranch, repo.GetPullRequestTargetBranch(ctx))
+
+	repo.Units = nil
+	prUnit, err := repo.GetUnit(ctx, unit.TypePullRequests)
+	assert.NoError(t, err)
+	prConfig := prUnit.PullRequestsConfig()
+	prConfig.DefaultTargetBranch = "branch2"
+	prUnit.Config = prConfig
+	assert.NoError(t, UpdateRepoUnit(ctx, prUnit))
+	repo.Units = nil
+	assert.Equal(t, "branch2", repo.GetPullRequestTargetBranch(ctx))
+}

models/repo/repo.go

@@ -613,16 +613,13 @@ func (repo *Repository) ComposeCompareURL(oldCommitID, newCommitID string) strin
 	return fmt.Sprintf("%s/%s/compare/%s...%s", url.PathEscape(repo.OwnerName), url.PathEscape(repo.Name), util.PathEscapeSegments(oldCommitID), util.PathEscapeSegments(newCommitID))
 }
 
-func (repo *Repository) ComposeBranchCompareURL(baseRepo *Repository, branchName string) string {
-	if baseRepo == nil {
-		baseRepo = repo
-	}
+func (repo *Repository) ComposeBranchCompareURL(baseRepo *Repository, baseBranch, branchName string) string {
 	var cmpBranchEscaped string
 	if repo.ID != baseRepo.ID {
 		cmpBranchEscaped = fmt.Sprintf("%s/%s:", url.PathEscape(repo.OwnerName), url.PathEscape(repo.Name))
 	}
 	cmpBranchEscaped = fmt.Sprintf("%s%s", cmpBranchEscaped, util.PathEscapeSegments(branchName))
-	return fmt.Sprintf("%s/compare/%s...%s", baseRepo.Link(), util.PathEscapeSegments(baseRepo.DefaultBranch), cmpBranchEscaped)
+	return fmt.Sprintf("%s/compare/%s...%s", baseRepo.Link(), util.PathEscapeSegments(baseBranch), cmpBranchEscaped)
 }
 
 // IsOwnedBy returns true when user owns this repository

models/repo/repo_unit.go

@@ -131,6 +131,7 @@ type PullRequestsConfig struct {
 	DefaultDeleteBranchAfterMerge bool
 	DefaultMergeStyle             MergeStyle
 	DefaultAllowMaintainerEdit    bool
+	DefaultTargetBranch           string
 }
 
 // FromDB fills up a PullRequestsConfig from serialized format.