[PATCH][MERGE] Improvements to is_ignored, take 2

[Johan Rydberg]

John A Meinel <john at arbash-meinel.com> writes:

> Also, right now, we only really use the PassThroughTransaction, which
> doesn't do anything, [...]

Related to this; why is keeping us back from changing the pass through
transactions identity map from a NullIdentityMap to a real caching one?

Probably needed for knits, to keep bzrlib from re-reading the knit index 
every time. 

~j