format-patch: use default email for generating message ids

format-patch: use default email for generating message ids

We try to generate a sane message id for cover letters and
threading by appending some changing bits to the front of
the user's email address. The current code parses the email
out of the results of git_committer_info, but we can do this
much more easily by just calling ident_default_email
ourselves.

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