Add fast-path to the bzr loader
Computing a delta from the currently stored snapshot for each revision is much more efficient than always reading every single file.
We intentionally don't handle the cases where renames or copies are involved. The complexity of the logic needed to figure out nested or otherwise conflicting results coupled with the lack of nice primitives in Breezy itself is really too much to be worth the effort, as revisions with renames are far from being the majority.
Loading lp:glamour
on my laptop before this change takes 59 minutes,
less than 3 minutes with this change.
Migrated from D7130 (view on Phabricator)