export doesn't allow overwriting current document (?)

[Caolán McNamara]

On Thu, 2019-02-07 at 15:19 +0200, Noel Grandin wrote:
> Given how we cache and re-read external stuff like images, perhaps at
> one point in time, we could end up reading from 
> the same file that we are writing to during the export process?

IIUC we write to a temp file during save/export and when all is
complete then replace the original file with that copy, so I don't
think that case arises (I think)