Add level-of-concern data to git-repo-structure output

Output from git-sizer(1) shows a level-of-concern meter for the outputted stats. This provides users with additional context regarding which metrics may be deserving of more addition. This would also be useful to add to git-repo-structure.

Example git-sizer(1) output:

| Name                         | Value     | Level of concern               |
| ---------------------------- | --------- | ------------------------------ |
| Overall repository size      |           |                                |
| * Trees                      |           |                                |
|   * Total size               |  5.18 GiB | **                             |
|   * Total tree entries       |   129 M   | **                             |
|                              |           |                                |
| Biggest objects              |           |                                |
| * Commits                    |           |                                |
|   * Maximum parents      [1] |    10     | *                              |
| * Trees                      |           |                                |
|   * Maximum entries      [2] |  1.16 k   | *                              |
| * Blobs                      |           |                                |
|   * Maximum size         [3] |  10.0 MiB | *                              |
|                              |           |                                |
| History structure            |           |                                |
| * Maximum tag depth      [4] |     2     | *                              |
|                              |           |                                |
| Biggest checkouts            |           |                                |
| * Maximum path length    [5] |   119 B   | *                              |