commit/commit-builder/inventory serialisation api friction

[Ian Clatworthy]

Robert Collins wrote:

> Sorting requires the full path; we might be able to do something with a
> decorated lines list at some point. Its down to about 0.4 of a second
> now, so the wins there are capped quite low.

But that's still a lot (10% of target say) in an incremental commit. So
I think it's great that you're putting ideas like this on the table for
debate. We won't get there unless we do.

Ian C.