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.

sys/src/cmd/git/export

@@ -41,7 +41,6 @@ for(c in $commits){
 		if(test -d $cp/tree)
 			bind $cp/tree b
 		
-		echo From $c
 		echo From: `{cat $cp/author}
 		echo Date: `{date -um `{mtime $cp/author | awk '{print $1}'}}
 		<$cp/msg awk '