mailmap: stop using the_repository

mailmap: stop using the_repository

The 'read_mailmap' and 'read_mailmap_blob' functions rely on the global
'the_repository' variable. Update both functions to accept a
'struct repository' parameter.

Update all callers to pass 'the_repository' to retain the current
behavior.

Signed-off-by: Burak Kaan Karaçay <bkkaracay@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>