add sys.exit(main()) to file_contents_sorter.py

pre_commit_hooks/file_contents_sorter.py

diff --git a/pre_commit_hooks/file_contents_sorter.py b/pre_commit_hooks/file_contents_sorter.py
index 6f13c98..1598d2d 100644
--- a/pre_commit_hooks/file_contents_sorter.py
+++ b/pre_commit_hooks/file_contents_sorter.py
@@ -12,6 +12,7 @@
 from __future__ import print_function
 
 import argparse
+import sys
 from typing import IO
 from typing import Optional
 from typing import Sequence
@@ -53,3 +54,7 @@
             retv |= ret_for_file
 
     return retv
+
+
+if __name__ == '__main__':
+    sys.exit(main())