git plugin's track-tags option isn't enough for gnulib's git-version-gen

When the `track-tags` option of the `git` plugin is enabled, it generates a slimmed-down `.git` directory enough for `git describe` and `git log` to succeed.

However gnulib's `git-version-gen` script relies on `git log .` which doesn't work, and returns an error like
```
fatal: unable to read tree 1bdaa2ce628b458ab6a7c9e03023a9aed221cf75
```

Start at the git plugin's `track-tags` handling and reproduce the failure with gnulib's `git-version-gen`, comparing `git describe`, `git log`, and `git log .`. Done means the slimmed-down `.git` directory supports the script without the reported missing-tree error.

Written by the indexing model from the issue text.