fix: diff a renamed pr file against its previous path

[undont]

Summary

• a renamed file in a PR review rendered as a whole-file add: GetFileVersions fetched one path at both refs, so the new path 404'd at the base sha and came back Missing: true, which reached the model as an empty old side; GetFileVersions now takes a base path beside the head path
• get_file_versions gains an optional previous_path, which the sidecar has read off the REST file list into PRFile.PreviousPath all along but wasn't used
• client.get_file_versions takes the file entry rather than a bare path, matching git.model's { path, previous_path } on the local side
• the blob cache keys on (owner, repo, ref, path), so the two sides cache separately with no change
• the demo fixture gains a renamed file (lua/monokai.lua -> lua/dracula.lua), the easiest way to eyeball this offline

Test plan

• make check clean