Test Cases

Add test files for the following cases

- if in a git repository
- output if not in a git repository
- on a repo with just 1 commit and no tags
- on a repo with just 1 commit and 1 tag
- on a repo with 1 commit and multiple tags pointing to same commit
- on a repo with 1 tag and multiple commits
- on a repo with multiple tags and multiple commits

No files or test entry points are named in the issue. Start by locating the Go CLI's existing test structure and its repository and tag handling, then add coverage for each listed repository state and verify that all expected outputs and failure behavior pass.

索引モデルが issue の本文から書いたものです。