t7415: adjust test for dubiously-nested submodule gitdirs for v2.20.x

In v2.20.x, Git clones submodules recursively by first creating the
submodules' gitdirs and _then_ "updating" the submodules. This can lead
to the situation where the clone path is taken because the directory
(while it exists already) is not a git directory, but then the clone
fails because that gitdir is unexpectedly already a directory.

This _also_ works around the vulnerability that was fixed in "Disallow
dubiously-nested submodule git directories", but it produces a different
error message than the one expected by the test case, therefore we
adjust the test case accordingly.

Note: as the two submodules "race each other", there are actually two
possible error messages, therefore we have to teach the test case to
expect _two_ possible (and good) outcomes in addition to the one it
expected before.

Note: this workaround is only necessary for the v2.20.x release train;
The behavior changed again in v2.21.x so that the original test case's
expectations are met again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
