endTransaction should take a transaction to be ended.

Parsing code in ROOT can lead to lookups which open a nested transactions. Ending
the current transaction in these cases is misleading because usually the
transaction that is expected to be ended is different.

Add an argument to endTransaction to disambiguate. Please note that this fixes
the regular cases of ROOT autoloading.

One of the remaining issues is autoloading of classes, which are passed in as
template arguments. The second remaining is autoloading of classes residing in
unnamed namespaces. Eg. MyNamespace::MyClass, where the diagnostic is issued upon
seeing that MyNamespace is unknown.