qt: Only override -datadir if different from the default

Fixes #3905.

src/qt/intro.cpp

@@ -189,6 +189,11 @@ void Intro::pickDataDirectory()
 
         settings.setValue("strDataDir", dataDir);
     }
+    /* Only override -datadir if different from the default, to make it possible to
+     * override -datadir in the bitcoin.conf file in the default data directory
+     * (to be consistent with bitcoind behavior)
+     */
+    if(dataDir != getDefaultDataDirectory())
         SoftSetArg("-datadir", GUIUtil::qstringToBoostPath(dataDir).string()); // use OS locale for path setting
 }