percentages during rereads are bogus

When rereading uncertain blocks the percentages reported are invalid:
```
re-reading 10 uncertain blocks
reread 1813.79% done in 00:00:00, expected time:00:00:00

re-reading 265 uncertain blocks
reread 409.43% done in 00:00:54, expected time:00:00:13

re-reading 262 uncertain blocks
reread 409.89% done in 00:00:53, expected time:00:00:13
```