Fix up X_KART_ATTACHMENTS checkout

Makes X_KART_ATTACHMENTS checkout work, to the extent that it is
currently implemented. Attached files can be checked out to the workdir,
and are now written to the index. Note that editing the files doesn't
yet affect the working copy diff.

kart/workdir.py

@@ -4,7 +4,6 @@
 import logging
 from enum import Enum, auto
 import functools
-from pathlib import Path
 import shutil
 import sys
 from kart.structure import RepoStructure
@@ -20,7 +19,6 @@
 from kart.diff_structs import Delta, DatasetDiff
 from kart.exceptions import (
     NotFound,
-    SubprocessError,
     NO_WORKING_COPY,
     translate_subprocess_exit_code,
 )
@@ -35,7 +33,6 @@
 from kart.tile import ALL_TILE_DATASET_TYPES
 from kart.tile.tile_dataset import TileDataset
 from kart.tile.tilename_util import (
-    remove_any_tile_extension,
     case_insensitive,
     PAM_SUFFIX,
 )
@@ -423,7 +420,9 @@ def reset(
             kart_attachments = os.environ.get("X_KART_ATTACHMENTS")
             if kart_attachments:
                 if base_tree and target_tree:
-                    self.update_file_diffs(base_tree, target_tree, workdir_index)
+                    self.write_attached_files_to_workdir(
+                        base_tree, target_tree, workdir_index, track_changes_as_dirty
+                    )
 
             for ds_path in ds_updates:
                 self._update_dataset_in_workdir(
@@ -441,26 +440,36 @@ def reset(
                 sess, self.repo.spatial_filter.hexhash
             )
 
-    def update_file_diffs(self, base_tree, target_tree, workdir_index):
+    def write_attached_files_to_workdir(
+        self, base_tree, target_tree, workdir_index, track_changes_as_dirty=False
+    ):
         """Get the deltas for attachment files and write them to the working copy."""
+        write_to_index = not track_changes_as_dirty
+
         repo = self.repo
         base_rs = RepoStructure(repo, base_tree)
         target_rs = RepoStructure(repo, target_tree)
         attachment_deltas = get_file_diff(base_rs, target_rs)
 
-        for filename, file_delta in attachment_deltas.items():
-            new_path = self.path / filename
-
-            # Delete the old file
-            if file_delta.old and new_path.is_file():
-                new_path.unlink()
+        for file_delta in attachment_deltas.values():
+            for filename in set(filter(None, (file_delta.old_key, file_delta.new_key))):
+                workdir_path = self.path / filename
+                if workdir_path.is_file():
+                    workdir_path.unlink()
+                if write_to_index:
+                    workdir_index.remove_all([filename])
 
-            # Create a new file
             if file_delta.new:
-                blob_data = repo[file_delta.new.value].data
-                new_path.write_bytes(blob_data)
-
-            # TODO: update workdir_index.
+                workdir_path = self.path / file_delta.new_key
+                blob_data = repo[file_delta.new_value].data
+                workdir_path.write_bytes(blob_data)
+                if write_to_index:
+                    workdir_index.add_entry_with_custom_stat(
+                        pygit2.IndexEntry(
+                            filename, pygit2.hash(blob_data), pygit2.GIT_FILEMODE_BLOB
+                        ),
+                        workdir_path,
+                    )
 
     def _diff_to_reset(
         self, ds_path, base_datasets, target_datasets, workdir_diff_cache, ds_filter