Only allow admins to rename default/protected branches (#33276)

Currently, anyone with write permissions to a repo are able to rename
default or protected branches.

This change follows
[GitHub's](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch)
design by only allowing repo/site admins to change these branches.
However, it also follows are current design for protected branches and
only allows admins to modify branch names == branch protection rule
names. Glob-based rules cannot be renamed by anyone (as was already the
case, but we now catch `ErrBranchIsProtected` which we previously did
not catch, throwing a 500).

services/repository/branch.go

@@ -416,6 +416,29 @@ func RenameBranch(ctx context.Context, repo *repo_model.Repository, doer *user_m
 		return "from_not_exist", nil
 	}
 
+	perm, err := access_model.GetUserRepoPermission(ctx, repo, doer)
+	if err != nil {
+		return "", err
+	}
+
+	isDefault := from == repo.DefaultBranch
+	if isDefault && !perm.IsAdmin() {
+		return "", repo_model.ErrUserDoesNotHaveAccessToRepo{
+			UserID:   doer.ID,
+			RepoName: repo.LowerName,
+		}
+	}
+
+	// If from == rule name, admins are allowed to modify them.
+	if protectedBranch, err := git_model.GetProtectedBranchRuleByName(ctx, repo.ID, from); err != nil {
+		return "", err
+	} else if protectedBranch != nil && !perm.IsAdmin() {
+		return "", repo_model.ErrUserDoesNotHaveAccessToRepo{
+			UserID:   doer.ID,
+			RepoName: repo.LowerName,
+		}
+	}
+
 	if err := git_model.RenameBranch(ctx, repo, from, to, func(ctx context.Context, isDefault bool) error {
 		err2 := gitRepo.RenameBranch(from, to)
 		if err2 != nil {