git/export: use 'date -f' instead of 'date -m'

The '-m' flag was added to date largely
to support git scripts. It predates the
tmdate code, which is why it exists, but
it's a recent enough addition that nothing
I'm aware of uses it, other than git.

As a result, it would be good to remove
it, so let's do that.

sys/src/cmd/git/export

@@ -42,7 +42,7 @@ for(c in $commits){
 			bind $cp/tree b
 		
 		echo From: `{cat $cp/author}
-		echo Date: `{date -um `{mtime $cp/author | awk '{print $1}'}}
+		echo Date: `{date -uf'WW, DD MMM YYYY hh:mm:ss Z' `{walk -em $cp/author}}
 		<$cp/msg awk '
 		NR == 1 {
 			n = ENVIRON["n"]