Porcelain archive - create archive from a tree

[mroczect]

Summary

Implement archive to export a tree as a tar or zip file.

Context

Useful for releases.

Tasks

• Create porcelain/src/admin/archive.rs.
• Resolve tree (commit).
• Walk tree and write files to an archive format (tar, zip).
• Document.

Acceptance Criteria

• Produces valid archive.

Dependencies

• Plumbing: rev_parse, ls_tree or recursive tree read, cat_file.