Fix JSF Migration applying changes on the main branch

Target platform: LTS 2017 (9.10).
Marketplace Packages: Nuxeo Web UI and Nuxeo JSF UI.

Steps to reproduce:

1. Create a schema s1 (id and prefix):

• Create a single valued "String" field string.

2. Create a document type dt1 (id and label):

• On the "Schema" tab, add s1.
• On the "Creation Layout" tab, add a row and drag and drop the string field (from the s1 schema).
• Set the "Widget Type" to "Single vocabulary" and select the continent vocabulary.

3. Commit and push your changes, using any commit message.

4. Go to the "Migration Services" and click "Start Migration".

5. Checking back on the schema s1 you’ll find that the type of the field string has been changed from "String" to "Directory". This behaviour is not desired: the migration should not apply any changes to the original branch.

Notes:

• Checking on "Branch Management", no new commits were added, so it is unclear how this change is happening.
• If instead of using a string field from an external schema (s1), a string field from the document type schema (dt1) is used, nothing is changed.