feat: provide a way to record and apply index changes.

These changes will then be applicable to an index that is created
from the written tree editor.

gix-merge/tests/fixtures/tree-baseline.sh

@@ -66,6 +66,20 @@ function baseline () (
     git -c merge.conflictStyle=$conflict_style merge-tree -z --write-tree --allow-unrelated-histories "$their_committish" "$our_committish" > "$merge_info" || :
     echo "$dir" "$conflict_style" "$their_commit_id" "$their_committish" "$our_commit_id" "$our_committish" "$merge_info" "$maybe_expected_tree" "$opt_deviation_message" >> ../baseline.cases
   fi
+
+  local index_path=.git/${conflict_style}-${our_committish}-${their_committish}.index
+  if [ ! -e $index_path ]; then
+    git checkout -f $our_committish
+    git merge -m m $their_committish || :
+    cp .git/index "$index_path"
+  fi
+
+  local index_path=.git/${conflict_style}-${their_committish}-${our_committish}.index
+  if [ ! -e $index_path ]; then
+    git checkout -f $their_committish
+    git merge -m m $our_committish || :
+    cp .git/index "$index_path"
+  fi
 )
 
 git init simple