Prefer EMAIL to username@hostname.

Prefer EMAIL to username@hostname.

The environment variable $EMAIL gives a better default of user's
preferred e-mail address than the hardcoded "username@hostname",
as it is understood by many existing programs.

We still honor GIT_*_EMAIL environment variables and user.email
configuration variable give them higher precedence, so that the
user can override $EMAIL or "username@hostname", as they are
likely to be more specific to the context of working on a
particular project.

Signed-off-by: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>