Use git to check for empty index.

lib/format-staged/io.rb

@@ -17,6 +17,17 @@ module FormatStaged
       output
     end
 
+    def get_status(*args)
+      puts "> #{args.join(' ')}" if @verbose
+      result = system(*args)
+
+      raise 'Failed to run command' if result.nil?
+
+      puts "? #{$CHILD_STATUS.exitstatus}" if @verbose
+
+      $CHILD_STATUS.exitstatus
+    end
+
     def pipe_command(*args, source: nil)
       puts (source.nil? ? '> ' : '| ') + args.join(' ') if @verbose
       r, w = IO.pipe

lib/format-staged/job.rb

@@ -36,7 +36,10 @@ module FormatStaged
 
       formatted = files.filter { |file| format_file file }
 
-      !formatted.empty?
+      return false unless formatted.size == files.size
+
+      quiet = @verbose ? [] : ['--quiet']
+      return get_status('git', 'diff-index', '--cached', '--exit-code', *quiet, 'HEAD') != 0
     end
 
     def repo_root
@@ -76,7 +79,7 @@ module FormatStaged
 
       if new_hash == file.src_hash
         info "File #{file.src_path} equal to comitted"
-        return false
+        return true
       end
 
       true