add argument separator before path

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/history/MercurialRepository.java

@@ -195,6 +195,7 @@ Executor getHistoryLogExecutor(File file, String sinceRevision)
         }
 
         if (!filename.isEmpty()) {
+            cmd.add("--");
             cmd.add(filename);
         }