avoid warning in case of mkdirs() races

Author: Vladimir Kotal

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

@@ -460,7 +460,8 @@ public void store(History history, Repository repository, String tillRevision) t
         history.strip();
 
         File histDataDir = new File(getRepositoryHistDataDirname(repository));
-        if (!histDataDir.isDirectory() && !histDataDir.mkdirs()) {
+        // Check the directory again in case of races (might happen in the presence of sub-repositories).
+        if (!histDataDir.isDirectory() && !histDataDir.mkdirs() && !histDataDir.isDirectory()) {
             LOGGER.log(Level.WARNING, "cannot create history cache directory for ''{0}''", histDataDir);
         }