Make gogit Repository.GetBranchNames consistent

nogogit GetBranchNames() lists branches sorted in reverse commit date
order. On the other hand the gogit implementation doesn't apply any
ordering resulting in unpredictable behaviour. In my case, the unit
tests requiring particular order fail

    repo_branch_test.go:24:
                Error Trace:
               ./gitea/modules/git/repo_branch_test.go:24
                Error:          elements differ

                                extra elements in list A:
                                ([]interface {}) (len=1) {
                                 (string) (len=6) "master"
                                }

                                extra elements in list B:
                                ([]interface {}) (len=1) {
                                 (string) (len=7) "branch1"
                                }

                                listA:
                                ([]string) (len=2) {
                                 (string) (len=6) "master",
                                 (string) (len=7) "branch2"
                                }

                                listB:
                                ([]string) (len=2) {
                                 (string) (len=7) "branch1",
                                 (string) (len=7) "branch2"
                                }
                Test:           TestRepository_GetBranches

To fix this, we sort branches based on their commit date in gogit
implementation.

Fixes: #28318

modules/git/repo_branch_gogit.go

@@ -8,6 +8,7 @@ package git
 
 import (
 	"context"
+	"sort"
 	"strings"
 
 	"github.com/go-git/go-git/v5/plumbing"
@@ -52,32 +53,46 @@ func (repo *Repository) IsBranchExist(name string) bool {
 
 // GetBranches returns branches from the repository, skipping "skip" initial branches and
 // returning at most "limit" branches, or all branches if "limit" is 0.
+// Branches are returned with sort of `-commiterdate` as the nogogit
+// implementation. This requires full fetch, sort and then the
+// skip/limit applies later as gogit returns in undefined order.
 func (repo *Repository) GetBranchNames(skip, limit int) ([]string, int, error) {
-	var branchNames []string
+	type BranchData struct {
+		name          string
+		committerDate int64
+	}
+	var branchData []BranchData
 
-	branches, err := repo.gogitRepo.Branches()
+	branchIter, err := repo.gogitRepo.Branches()
 	if err != nil {
 		return nil, 0, err
 	}
 
-	i := 0
-	count := 0
-	_ = branches.ForEach(func(branch *plumbing.Reference) error {
-		count++
-		if i < skip {
-			i++
-			return nil
-		} else if limit != 0 && count > skip+limit {
+	_ = branchIter.ForEach(func(branch *plumbing.Reference) error {
+		obj, err := repo.gogitRepo.CommitObject(branch.Hash())
+		if err != nil {
+			// skip branch if can't find commit
 			return nil
 		}
 
-		branchNames = append(branchNames, strings.TrimPrefix(branch.Name().String(), BranchPrefix))
+		branchData = append(branchData, BranchData{strings.TrimPrefix(branch.Name().String(), BranchPrefix), obj.Committer.When.Unix()})
 		return nil
 	})
 
-	// TODO: Sort?
+	sort.Slice(branchData, func(i, j int) bool {
+		return !(branchData[i].committerDate < branchData[j].committerDate)
+	})
+
+	var branchNames []string
+	maxPos := len(branchData)
+	if limit > 0 {
+		maxPos = min(skip+limit, maxPos)
+	}
+	for i := skip; i < maxPos; i++ {
+		branchNames = append(branchNames, branchData[i].name)
+	}
 
-	return branchNames, count, nil
+	return branchNames, len(branchData), nil
 }
 
 // WalkReferences walks all the references from the repository