Write files atomically

Author: keith

Sources/swift-format/Run.swift

@@ -96,7 +96,8 @@ func formatMain(
       buffer.flush()
       try localFileSystem.writeFileContents(
         AbsolutePath(assumingFileURL.path, relativeTo: AbsolutePath(cwd)),
-        bytes: buffer.bytes
+        bytes: buffer.bytes,
+        atomically: true
       )
     } else {
       try formatter.format(source: source, assumingFileURL: assumingFileURL, to: &stdoutStream)