Merge wd validation tests against index not HEAD

Validating the workdir should not compare HEAD to working
directory - this is both inefficient (as it ignores the cache)
and incorrect.  If we had legitimately allowed changes in the
index (identical to the merge result) then comparing HEAD to
workdir would reject these changes as different.  Further, this
will identify files that were filtered strangely as modified,
while testing with the cache would prevent this.

Also, it's stupid slow.

src/merge.c

@@ -2330,7 +2330,7 @@ done:
 
 static int merge_check_workdir(size_t *conflicts, git_repository *repo, git_index *index_new, git_vector *merged_paths)
 {
-	git_tree *head_tree = NULL;
+	git_index *index_repo = NULL;
 	git_diff *wd_diff_list = NULL;
 	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
 	int error = 0;
@@ -2341,9 +2341,6 @@ static int merge_check_workdir(size_t *conflicts, git_repository *repo, git_inde
 
 	opts.flags |= GIT_DIFF_INCLUDE_UNTRACKED;
 
-	if ((error = git_repository_head_tree(&head_tree, repo)) < 0)
-		goto done;
-
 	/* Workdir changes may exist iff they do not conflict with changes that
 	 * will be applied by the merge (including conflicts).  Ensure that there
 	 * are no changes in the workdir to these paths.
@@ -2351,13 +2348,13 @@ static int merge_check_workdir(size_t *conflicts, git_repository *repo, git_inde
 	opts.pathspec.count = merged_paths->length;
 	opts.pathspec.strings = (char **)merged_paths->contents;
 
-	if ((error = git_diff_tree_to_workdir(&wd_diff_list, repo, head_tree, &opts)) < 0)
+	if ((error = git_diff_index_to_workdir(&wd_diff_list, repo, index_repo, &opts)) < 0)
 		goto done;
 
 	*conflicts = wd_diff_list->deltas.length;
 
 done:
-	git_tree_free(head_tree);
+	git_index_free(index_repo);
 	git_diff_free(wd_diff_list);
 
 	return error;