Avoid showing the 'overwrite changes' dialog as a temporary workaround for #6437

The "overwrite changes" dialog that was introduced as part of the file watchers feature is not fully reliable on Mac, and is showing up occasionally when there appears to have actually been no change on disk. For now, we're simply disabling this dialog; in most cases we will still have warned the user (via the "external changes" dialog) that the file has been updated on disk, and in the small number of cases where they would have gotten the "overwrite changes" dialog, the behavior is no worse than it was before the file watchers feature.

Note that this is targeting the release branch.