Deleting a file with changes pops up dialog twice

[njx]

1. Open a file
2. Make some edits without saving
3. In the file tree, delete the file

Result: The "Do you want to keep changes" dialog appears to pop up twice in succession.

[njx]

This might well have always been in the product, but became more obvious when we added the dialog open animation. It might be similar to other issues we had with the way the file sync manager handles external changes.

[peterflynn]

Reviewed - low priority for 1.0, to fix along with #6712 if possible.

Ideally in this case we'd just suppress the dialog animation when FileSyncManager restarts, so there's no visible change.