Clean up blob diff path

Previously when diffing blobs, the diff code just ran with a NULL
repository object. Of course, that's not necessary and the test
for a NULL repo was confusing. This makes the blob diff run with
the repo that contains the blobs and clarifies the test that it
is possible to be diffing data where the path is unknown.

src/diff_output.c

@@ -151,7 +151,8 @@ static int update_file_is_binary_by_attr(
 {
 	const char *value;
 
-	if (!repo)
+	/* because of blob diffs, cannot assume path is set */
+	if (!file->path || !strlen(file->path))
 		return 0;
 
 	if (git_attr_get(&value, repo, 0, file->path, "diff") < 0)
@@ -1028,6 +1029,7 @@ int git_diff_blobs(
 	diff_delta_context ctxt;
 	git_diff_delta delta;
 	git_blob *new, *old;
+	git_repository *repo;
 
 	new = new_blob;
 	old = old_blob;
@@ -1038,8 +1040,15 @@ int git_diff_blobs(
 		new = swap;
 	}
 
+	if (new)
+		repo = git_object_owner((git_object *)new);
+	else if (old)
+		repo = git_object_owner((git_object *)old);
+	else
+		repo = NULL;
+
 	diff_delta_init_context(
-		&ctxt, NULL, options, GIT_ITERATOR_TREE, GIT_ITERATOR_TREE);
+		&ctxt, repo, options, GIT_ITERATOR_TREE, GIT_ITERATOR_TREE);
 
 	/* populate a "fake" delta record */