Fuga "Lass mich gehn, denn dort kommt meine Mutter her"

Johann Georg Schübler (1720 (?) - 1755) was a German engraver and organist, who belonged to the Schübler family of artisians from Zella in the Turingian forrest. He studied with Bach in Leipzig during the early 1740s and spent the rest of his life as an organist and a teacher Mehlis. Nowadays Schübler is primarily known as the engraver of the " Sechs Choräle von verschiedener Art..." (BWV 645-650) " In Verlegung Joh. Georg Schübler zu Zella am Thüringer Walde". However, he also composed some music himself. I know of one Trio for organ and the of the fugue presented here today.

Now, the fugue "Lass mich gehn, denn dort kommt meine Mutter her" is a strange case. It has the same theme (and counter subject) as the fugue BWV587 by Johann Sebastian Bach. Is it a composition exercise set by Bach? Is the title (which translates as "Let me go, for there comes my mother") Schübler's own idea? Or was it an existing song and is Bach's fugue then also based on that (children's) song?
It's a fugue in two voices, without pedal-part. So, for anyone (like me) to whom BWV587 poses too many technical problems, here's your chance to play a fugue, based on that famous theme.

Score available here:
http://partitura.org/index.php/johann-georg-schubler-fuga-lass-mich-gehn-denn-dort-kommt-meine-mutter-her/