Here is a case leading to wrong behavior :

1. Make an edit to the "content" property of a file via a script.

2. The script crashed.  Thus, the database is neither committed, nor
   rolled back.

3. Ran the script again, this time to completion, and with a
   database commit.

Yet, the contents of the file did not change. So, the state of the
database after a commit does not match the expected state!

The problem is that Roundup is trying to avoid recording a
transaction hook for the same file twice (which is good, of course),
but based that decision on whether the temporary file already existed
(which is bad, since it exists in the crash scenario above).

Fixed with the attached patch.