check-mailmap: add options for additional mailmap sources

check-mailmap: add options for additional mailmap sources

The git check-mailmap command reads the mailmap from either the default
.mailmap location and then from the mailmap.blob and mailmap.file
configurations.

A following change to git send-email will want to support new
configuration options based on the configured identity. The
identity-based configuration and options only make sense in the context
of git send-email.

Expose the read_mailmap_file and read_mailmap_blob functions from
mailmap.c.  Teach git check-mailmap the --mailmap-file and
--mailmap-blob options which load the additional mailmap sources.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>