Sparse revlog requirements

[Gregory Szorc]

I'm currently reviewing and landing Boris's series to enable sparse revlog
by default. I think the functionality is stable enough to be enabled by
default.

The way it is currently implemented, sparse revlogs are an additional
repository requirement. The "revlogv1" requirement denotes version 1
revlogs and the "sparserevlog" requirement denotes that revlogs are sparse.
Sparse revlog functionality is enabled if the repository requirement is
present.

I'd like to put forward a proposal to change this behavior before the 4.9
release. And by "change" I mean "change for new repos but likely maintain
BC with the existing behavior for existing repos [if it makes sense]."

At the very minimum, I think we should introduce a revlog flag that denotes
the revlog as sparse. We have 16 bits to use and we're only using 2 bits
for "inline" and "generaldelta." I propose we allocate the next bit to
denote sparse I/O.

Having a revlog flag to denote sparse forces the "is sparse or not" to live
closer to the data. It allows revlog opening to not directly care about
repository requirements. It gives us flexibility to toggle sparse behavior
on a per-revlog basis (if that makes sense).

This change would align the behavior of sparse revlogs with generaldelta
revlogs.

Thoughts?
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