Protect default branch against deletion (#11115)

Although default branch is not offered for deletion in the templates, we need to prevent it both at the router level and in the pre-receive hook.

Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>

routers/repo/branch.go

@@ -57,8 +57,12 @@ func Branches(ctx *context.Context) {
 // DeleteBranchPost responses for delete merged branch
 func DeleteBranchPost(ctx *context.Context) {
 	defer redirect(ctx)
-
 	branchName := ctx.Query("name")
+	if branchName == ctx.Repo.Repository.DefaultBranch {
+		ctx.Flash.Error(ctx.Tr("repo.branch.default_deletion_failed", branchName))
+		return
+	}
+
 	isProtected, err := ctx.Repo.Repository.IsProtectedBranch(branchName, ctx.User)
 	if err != nil {
 		log.Error("DeleteBranch: %v", err)