PatchOrderGenerator: Preserve csv line order

Order the csv files by the verified date of the first entry.  Then
append entries together from all files preserving the order within each
file.  This replaces the previous sort by verified date which is not
sufficient because the date does not always increase monotonically
within a csv file.

Change-Id: I8715a0fbb9c48ea8592f9d34870326f599c521a2