export: only bind commit if it exists

This fixes an edge case with the first commit,
where no parent exists. In that case, we want
to diff against an empty directory.

export

@@ -30,14 +30,16 @@ m=$#commits
 mntgen /mnt/scratch >[2]/dev/null || status=''
 for(c in $commits){
 	cp=`{git/query -p $c}
-	pp=`{git/query -p $c'^'}
+	pp=`{git/query -p $c'~'}
 	fc=`$nl{git/query -c $c~ $c | sed 's/^..//'}
 
 	@{
 		rfork n
 		cd /mnt/scratch
-		bind $pp/tree a
-		bind $cp/tree b
+		if(test -d $pp/tree)
+			bind $pp/tree a
+		if(test -d $cp/tree)
+			bind $cp/tree b
 		
 		echo From $c
 		echo From: `{cat $cp/author}