Resolve a relative link target against the artifact's directory and return an absolute GitHub URL.
Original link target (e.g. ../templates/foo.md)
../templates/foo.md
Repo-relative path of the artifact
When true, produce a raw.githubusercontent URL (for images)
Absolute URL (or the original target for anchors/absolute links)
Resolve a relative link target against the artifact's directory and return an absolute GitHub URL.