mailmap: support reading mailmap from blobs

mailmap: support reading mailmap from blobs

In a bare repository, there isn't a simple way to respect an
in-tree mailmap without extracting it to a temporary file.
This patch provides a config variable, similar to
mailmap.file, which reads the mailmap from a blob in the
repository.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>