Auto-complete for grub-reboot provides invalid options, most of which fail silently

[Ricardo Abreu]

Hitting tab on `grub-reboot` gives:

```
$ grub-reboot [tab]
--id
Advanced options for Ubuntu
Ubuntu
Ubuntu, with Linux 4.15.0-108-generic
Ubuntu, with Linux 4.15.0-108-generic (recovery mode)
Ubuntu, with Linux 4.15.0-109-generic
Ubuntu, with Linux 4.15.0-109-generic (recovery mode)
```

But

```
$ grub-reboot --id
Unrecognized option `--id'
[...]
```

And the menu items starting with "Ubuntu, with Linux 4.15.0-" are actually in the submenu "Advanced options for Ubuntu". So, saying `grub-reboot "Ubuntu, with Linux 4.15.0-108-generic", does not have the desired effect. One needs to say instead `grub-reboot "Advanced options for Ubuntu>Ubuntu, with Linux 4.15.0-108-generic"`.

Additionally, `grub-reboot` accepts those and other meaningless parameters without complaint:

```
$ sudo grub-reboot nonsense && echo success
success
```

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: grub2-common 2.02-2ubuntu8.15
ProcVersionSignature: User Name 4.15.0-109.110-generic 4.15.18
Uname: Linux 4.15.0-109-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
Date: Fri Jul 3 16:35:28 2020
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)