Sorting branch listing by "most recently changed first" puts unscanned branches first

[James Henstridge]

The following branch listing uses the "most recently changed first" order:

https://code.edge.launchpad.net/storm/+branches?field.lifecycle=CURRENT&field.sort_by=most+recently+changed+first

However, the first entry in the listing (~sidnei/storm/mssql-support) says it was last modified 28 weeks ago, which is a lot older than the second entry.

The difference is that the first item has not been scanned: it appears to be a branch hosted branch that hasn't been pushed to yet. The NULL in its last_scanned field is probably sorting less than the non-NULL values for all the other branches.

It would be less confusing if the sorting was done by the same value as used in the "last modified" column (i.e. date_last_modified).