When generating diffs from FileDiffs or results for the FileDiff

resource, we were using the database's default sort order. For MySQL and

MariaDB, this is going to be the primary key, but that's not guaranteed

for Postgres. For Postgres, we can get an entirely different order.
This can pose a problem for diffs that are sensitive to file orders,

namely Mercurial, which contains identifying information for the commit

in the first file. We need these to be in the insert order, as that's

the order in which we parse from the sourcediff.
We now explicitly sort these by the primary key in these cases. We're

not setting this as the default sort order at this time, in order to

avoid any surprises elsewhere, and to be explicit about the order we

know we need in these particular cases.

Unit tests pass.
Verified this fix with a customer who had previously encountered

this issue.