!!! WIP !!! reminder of what is still left to do ...
!!! WIP !!! submodules: extend lib-submodule-update.sh for recursive update

TODO: test correct behavior with submodule content modifications

Add two new test helper functions (test_submodule_recursive_switch() and
test_submodule_forced_recursive_switch()) to simplify testing all relevant
combinations of recursive submodule update scenarios without having to add
them to a test of each work tree manipulating command over and over again.

While at it add an omitted "submodule update" followed by a test of the
proper submodule contents to the test_submodule_forced_switch() function
added in 42639d2317ac.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>