Refactor the FileRevisionIterator

• rename it as CSVRevisionIterator
• make it take an iterable as input instead of a file name
• make it a true iterator (iter)
• refactor the main method to simplify it

The expected input iterable should generate 3-tuples, corresponding the
the 3 columns of the originally expected CSV file (revision sha1, date,
directory sha1).

This makes the class much easier to test.