help me understand test_fetch_no_inventory_revision

[Aaron Bentley]

Robert Collins wrote:
>> I'm not sure what this is meant to test;

It's meant to test that fetching revisions when the source inventory has
no revision_id stored works correctly.  I expect I put it in when I was
working on KnitFormat2 or 3.

>> its being triggered by the
>> InterPack fetch optimiser, which fails currently. But I'm not sure why
>> its failing, and the monkey patching of deserialise_inventory seems
>> unrelated to what the optimisers do (any native format optimiser is
>> unlikely to deserialise the full inventory ...)

A converter that converts from one inventory format to another will use
it.  Note that Inventory format 6 guarantees a revision, while inventory
format 5 does not.

Aaron