history: prevent overwriting of positions between multiple instances

Whenever a buffer is closed, check whether the positions file on disk
was modified, and if so, reload it.  Then update the position for the
closed buffer and write out the positions file to disk.

Signed-off-by: Brand Huntsman <alpha@qzx.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>
3 files changed