Second hierarchy migration fails due to index name clash

[rcanand]

I use closure tree to create hierarchies for two models in my app. The first one succeeds, but the second one fails, due to the generated index name conflict ("anc_desc_idx" and "desc_idx") . Renaming these indices to be model specific ("tag_anc_desc_idx" and "tag_desc_idx", for instance) is a workaround.
Expected: The index name generated to include the model name to disambiguate indices across models.

[seuros]

The bug was introduced in d040640#diff-1b9222a399a8e5870dd9f6852b9c1c27

@mceachen , we should fix this.

[seuros]

@rcanand do you mind sending a PR ?

[seuros]

@rselk, do you want to take a stab for this fix ?

[mceachen]

I'll be able to do a patch release tonight with a fix. Thanks for reporting
the issue!

[mceachen]

(To clarify, I'll pull your branch when you've got it ready)

[rselk]

pr sent for this #129