test_backport.py: Clean up .gitmodules before starting tests.

In setup_unusual_submodule_path_change(), call
self.cleanup_unusual_submodule_change() after calling
self.setup_visualeditor_repo() to ensure that any commits that have
been merged into VisualEditor/VisualEditor are reflected with an
update to .gitmodules in extensions/VisualEditor.

cleanup_unusual_submodule_change() changes:

• announce the boundaries of the operation
• handle the case where no .gitmodules update is needed.

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Related MRs:

• !249 (merged) (This MR)
• !248 (merged)
• master

Depends-On: !248 (merged)