Remove confusing TrimPrefix(... git.BranchPrefix) (#20369)

Make Repository.GetDefaultBranch return the real branch name, instead of the ref name. Then there is no need to do TrimPrefix for repo.DefaultBranch

modules/git/repo_branch.go

@@ -7,6 +7,7 @@ package git
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"strings"
 )
@@ -72,7 +73,14 @@ func (repo *Repository) SetDefaultBranch(name string) error {
 // GetDefaultBranch gets default branch of repository.
 func (repo *Repository) GetDefaultBranch() (string, error) {
 	stdout, _, err := NewCommand(repo.Ctx, "symbolic-ref", "HEAD").RunStdString(&RunOpts{Dir: repo.Path})
-	return stdout, err
+	if err != nil {
+		return "", err
+	}
+	stdout = strings.TrimSpace(stdout)
+	if !strings.HasPrefix(stdout, BranchPrefix) {
+		return "", errors.New("the HEAD is not a branch: " + stdout)
+	}
+	return strings.TrimPrefix(stdout, BranchPrefix), nil
 }
 
 // GetBranch returns a branch by it's name