The email addresses in the records read from the .mailmap file are
downcased very early, and then used to match against e-mail
addresses in the input.  Because we do use case insensitive version
of string list to manage these entries, there is no need to do this,
and worse yet, downcasing the rewritten/canonical e-mail read from
the .mailmap file loses information.

Stop doing that, and also make the string list used to keep multiple
names for an mailmap entry case insensitive (the code that uses the
list, lookup_prefix(), expects a case insensitive match).

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>