…s are provided

…s are specified

When explicit roots and reference filters are both provided, we want to make
sure that the optimisation don't kick in. As we still want to iterate
over all references in order to apply the filters.

Here is an example with git-sizer being run with explicit roots and reference filters
```
git-sizer main~20^{tree} main~10^{tree} --tags --branches
```

We printed out the reference count, even when explicit roots were
provided.
```
git-sizer master~50^{tree} 'HEAD@{1.week.ago}' -v
[TRUNCATED_OUTPUT]
| * References                 |           |                                |
|   * Count                    |   187     |                                |
|     * Ignored                |   187     |                                |
|                              |           |                                |
[TRUNCATED_OUTPUT]
```

This is not very useful, as the reference count depends on the list of references
extracted by running `git for-each-ref`.

This commit changes the output to not print the reference count when
explicit roots and no reference filters are provided to git-sizer

The reference count is still displayed when the -j flag is passed to
git-sizer.
```
git-sizer master~50^{tree} 'HEAD@{1.week.ago}' -v -j
[TRUNCATED_OUTPUT]
    "reference_count": 15,
    "reference_groups": {
        "ignored": 15
    },
[TRUNCATED_OUTPUT]
```

We don't want to print the reference count for json v1, when explicit
root are not provided git-sizer