Git grep command not interpreting flags correctly when an argument

[Sam Houston <sehouston3@xxxxxxxxx>]

In a `bash` shell, in some empty directory, I can run the commands:

```bash
git init
echo "pattern" > file.txt
FLAGS="--untracked --color"
git grep $FLAGS pattern
```

And see the output:

```
file.txt:pattern
```

But in a `zsh` shell, when I run the same commands, I get the following
error:

```
error: unknown option `untracked --color'
```

The error only seems to occur when I pass in with more than one `--` in the
`FLAGS` variable.

My `zsh --version` is `zsh 5.3 (x86_64-apple-darwin18.0)`.

Let me know if I am doing something wrong, or if you need more information.

Thanks,

Sam