mailmap doc + tests: document and test for case-insensitivity

mailmap doc + tests: document and test for case-insensitivity

Add documentation and more tests for case-insensitivity. The existing
test only matched on the E-Mail part, but as shown here we also match
the name with strcasecmp().

This behavior was last discussed on the mailing list in the thread
starting at [1]. It seems we're keeping it like this, so let's
document it.

1. https://lore.kernel.org/git/87czykvg19.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>