Artifact stanza can't fetch a specific file from a git repository

[go-getter](https://github.com/hashicorp/go-getter) allows [fetching a subdirectory](https://github.com/hashicorp/go-getter#subdirectories), but this doesn't work if you specify a file instead of a directory.

Being able to fetch individual files from a git repository would be very useful for use cases where configuration files, templates etc. are versioned and stored in a VCS.

No files or tests are named. Start by tracing how the artifact stanza handles go-getter Git sources, then identify the existing subdirectory-fetching path. Done means an artifact source can retrieve one specified file from a Git repository, with coverage for that behavior.

Written by the indexing model from the issue text.