editor: remove out of sync branch before checkout again

If a branch was deleted from server, sometimes it is not reflected
on local copy. Therefore, we need to remove the branch with same
name if it is out of sync and then checkout to correct version.

models/errors/repo.go

@@ -59,3 +59,16 @@ func IsMirrorNotExist(err error) bool {
 func (err MirrorNotExist) Error() string {
 	return fmt.Sprintf("mirror does not exist [repo_id: %d]", err.RepoID)
 }
+
+type BranchAlreadyExists struct {
+	Name string
+}
+
+func IsBranchAlreadyExists(err error) bool {
+	_, ok := err.(BranchAlreadyExists)
+	return ok
+}
+
+func (err BranchAlreadyExists) Error() string {
+	return fmt.Sprintf("branch already exists [name: %s]", err.Name)
+}

models/repo_editor.go

@@ -21,6 +21,7 @@ import (
 
 	git "github.com/gogits/git-module"
 
+	"github.com/gogits/gogs/models/errors"
 	"github.com/gogits/gogs/pkg/process"
 	"github.com/gogits/gogs/pkg/setting"
 )
@@ -92,13 +93,29 @@ func (repo *Repository) UpdateRepoFile(doer *User, opts UpdateRepoFileOptions) (
 		return fmt.Errorf("UpdateLocalCopyBranch [branch: %s]: %v", opts.OldBranch, err)
 	}
 
+	repoPath := repo.RepoPath()
+	localPath := repo.LocalCopyPath()
+
 	if opts.OldBranch != opts.NewBranch {
+		// Directly return error if new branch already exists in the server
+		if git.IsBranchExist(repoPath, opts.NewBranch) {
+			return errors.BranchAlreadyExists{opts.NewBranch}
+		}
+
+		// Otherwise, delete branch from local copy in case out of sync
+		if git.IsBranchExist(localPath, opts.NewBranch) {
+			if err = git.DeleteBranch(localPath, opts.NewBranch, git.DeleteBranchOptions{
+				Force: true,
+			}); err != nil {
+				return fmt.Errorf("DeleteBranch [name: %s]: %v", opts.NewBranch, err)
+			}
+		}
+
 		if err := repo.CheckoutNewBranch(opts.OldBranch, opts.NewBranch); err != nil {
 			return fmt.Errorf("CheckoutNewBranch [old_branch: %s, new_branch: %s]: %v", opts.OldBranch, opts.NewBranch, err)
 		}
 	}
 
-	localPath := repo.LocalCopyPath()
 	oldFilePath := path.Join(localPath, opts.OldTreeName)
 	filePath := path.Join(localPath, opts.NewTreeName)
 	os.MkdirAll(path.Dir(filePath), os.ModePerm)