Xgit.Repository.WorkingTree.dir_cache/1 output is potentially large.

[scouten]

• Add label: "TO DO"
• Add label: "good first issue" or "help wanted" if appropriate
• Add link to this issue in a comment near the TO DO comment

Describe What Is Incomplete
The function Xgit.Repository.WorkingTree.dir_cache/1 returns a full copy of the Xgit.Core.DirCache struct, complete with all of its entries. That's fine for a small repo, but risky for a potentially large repo.

Definition of Done
Think through how to pass a snapshot of the DirCache struct without overwhelming the VM message-passing mechanism. (Consider a repo with many thousands of files in the index.)