workspace: seed adopted worktree from its own Git index

## Summary

Allows `workspace add --existing-git-worktree` to adopt a clean detached worktree even when the source workspace has unresolved Git index entries. File-state seeding now reads the destination worktree's index, preserving the isolation Git provides between linked worktrees.

Adds a regression case with a conflicted source index and a clean destination index.

## Test Plan

- Targeted workspace adoption regression test passes
- Modified Rust files pass `rustfmt --check`

No-Auto-Fix

working-copy: parallelize seeded state ordering

## Summary

Reduces initialization latency when adopting large populated Git worktrees by sorting seeded file state across the Rayon worker pool. The existing repository-path ordering remains unchanged, preserving the correctness required by working-copy state merges.

## Test Plan

- ✅ `cargo test -p jj-lib test_fsmonitor --no-default-features --features git`
- ✅ `cargo test -p jj-cli test_workspace_add_adopts_existing_git_worktree`
- ✅ Treehouse warm benchmark improved from 6.03 seconds to 4.02 seconds

No-Auto-Fix

watchman: allow restoring the working-copy clock

working-copy: preserve seeded state ordering

Git index order differs from JJ's component-wise RepoPath order for prefix-related names. Keeping that order in the seeded file-state table makes later full scans misclassify existing files as deleted, which breaks recovered overlay workspaces.

release: 0.39.0

Signed-off-by: Austin Seipp <aseipp@pobox.com>

release: 0.38.0

release: 0.37.0

Signed-off-by: Austin Seipp <aseipp@pobox.com>

release: 0.36.0

release: 0.35.0

release: 0.34.0

Signed-off-by: Austin Seipp <austin@ersc.io>
Signed-off-by: Austin Seipp <aseipp@pobox.com>