[Core][Cont] TRefArray: force the loading of the referenced branch when accessing by index

[dpiparo]

This PR fixes ROOT-3594

Sister PR in roottest: https://github.com/root-project/roottest/pull/1166

Thanks a lot to @ferdymercury for the initial stab https://github.com/root-project/root/pull/15932

[github-actions[bot]]

Test Results

    12 files      12 suites   2d 19h 21m 24s :stopwatch:  3 028 tests  2 997 :white_check_mark: 0 :zzz:  31 :x: 31 261 runs  30 939 :white_check_mark: 0 :zzz: 322 :x:

For more details on these failures, see this check.

Results for commit b6cb0951.