Modify "git rev-parse --verify" args in "has_ref?"

Per the `git-rev-parse(1)` manpage:

> --verify
> Verify that exactly one parameter is provided, and that it can be turned into a raw 20-byte SHA-1 that can be used to access the object database. If so, emit it to the standard output; otherwise, error out.
>
> If you want to make sure that the output actually names an object in your object database and/or can be used as a specific type of object For example, git rev-parse "$VAR^{commit}" will make sure $VAR names an existing object that is a commit-ish (i.e. a commit, or an annotated tag that points at a commit).

That actually means that:
```
git rev-parse --verify af8e768e2bd3b4398bca033998f83b0eb8874914
```
will _always_ return the SHA-1 hash — regardless of whether or not that's actually a valid reference!

Thus, when `GitDownloadStragtegy::update_repo` tries to check `has_ref?`, it mistakenly succeeds, and doesn't actually do a `git fetch origin`.

The fix is to use:
```
git rev-parse --verify "af8e768e2bd3b4398bca033998f83b0eb8874914^{commit}"
```
Ruby has a weird thing where globals and ivars can be interpolated without braces. I've been removing occurrences of it when I have another reason to touch a line, so I'll tweak this when I pull.