Make git-archimport log entries more consistent

Make git-archimport log entries more consistent

When appending the "git-archimport-id:" line to the end of log entries,
git-archimport would use two blank lines as a separator when there was no
body in the arch log (only a Summary: line), and zero blank lines when there
was a body (making it hard to see the break between the actual log message
and the git-archimport-id: line).

This patch makes git-archimport generate one blank line as a separator in all
cases.

Signed-off-by: Junio C Hamano <gitster@pobox.com>