t1309: document cases where we would want early config not to die()

t1309: document cases where we would want early config not to die()

Jeff King came up with a couple examples that demonstrate how the new
read_early_config() that looks harder for the current .git/ directory
could die() in an undesirable way.

Let's add those cases to the test script, to document what we would like
to happen when early config encounters problems.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>