When format 5 isn't format 5

[Aaron Bentley]

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Hi all,

In updating changeset code, I've discovered that our inventory format
has changed, without changing its version number.

Specifically, new format 5 inventories contain a revision ID, and older
ones do not.  It looks like this change was introduced by Robert's patch
to get rid of the trailing revision-id in basis-inventory filenames.  I
presume that it was intended that only basis inventories would include
revision-ids, but it's also been applied inventories in the inventory weave.

Revision testaments contain the inventory hash, which I believe is just
the inventory XML SHA1 hash.  This means that fixing existing
repositories would break signatures.

Among other things, this strongly suggests that our unit tests should
include a test to make sure we get a reproduceable testament when we
commit a revision with fixed values.

Aaron
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