fix the eden start integration tests

Summary:
My changes to command line argument parsing in D12927803 conflicted with one
of the new tests added in D10863987.

My changes made `edenfs` and `fake_edenfs` reject any trailing non-option
arguments.  The new test runs `fake_edenfs` with an extra
`--commandArgumentsLogFile` argument asking it to log all non-option arguments
to a file.

This diff updates `fake_edenfs` to allow non-option arguments when
`--commandArgumentsLogFile` is specified.

Reviewed By: strager

Differential Revision: D13014079

fbshipit-source-id: 82ec5fd758716c2b66ac0738b7aacdf884f31233

eden/integration/helpers/fake_edenfs.cpp

@@ -324,7 +324,7 @@ int main(int argc, char** argv) {
   }
 #endif
 
-  if (argc != 1) {
+  if (argc != 1 && FLAGS_commandArgumentsLogFile.empty()) {
     fprintf(stderr, "error: unexpected trailing command line arguments\n");
     return 1;
   }