Versioning must not rely on user gitignore file

Specially on `com.nuxeo.studio.core.versioning.VersioningGitLocalImpl`, jgit is taking into account the user local `gitignore` global configuration file.

The side effect is that some tests might not work depending on the user environment and it is very hard to be identified.

For instance, the `.metadata` folder can be ignored in a global file as it's an Eclipse project userland file.

Related to: https://bugs.eclipse.org/bugs/show_bug.cgi?id=488777