git git<tab> completes non-existent command `git gitk`

[Anthony Sottile <asottile@xxxxxxxxx>]

easiest to reproduce is with docker

```dockerfile
FROM ubuntu:focal
RUN : \
    && apt-get update \
    && DEBIAN_FRONTEND=noninteractive apt-get install -y
--no-install-recommends \
        bash-completion \
        git \
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/*
```

```console
$ docker run --rm -ti test bash
root@23e691ecc7ba:/# [ -f /etc/bash_completion ] && . /etc/bash_completion
root@23e691ecc7ba:/# git gitk
```

(I typed git git<tab>)

```console
$ git gitk
git: 'gitk' is not a git command. See 'git --help'.
```

this is a bit annoying because I have some aliases/commands for git-github-*

the git version I have is 2.25.1:

```console
$ git --version
git version 2.25.1
```

Anthony