git/export: make output pipable to /bin/mail

git/export: make output pipable to /bin/mail

git/export *almost* produces output that can be
emailed with upas using

	git/export $commit | mail maintainer@site.com

but, the

	From: commit-id date

line that git generates trips it up. Luckily,
'git am' doesn't seem to care much if that line
is missing, so we can simply omit it with no issue.