revlog: support reading generaldelta revlogs
Generaldelta is a new revlog global flag. When it's turned on, the base field
of each revision entry holds the deltaparent instead of the base revision of
the current delta chain.
This allows for great potential flexibility when generating deltas, as any
revision can serve as deltaparent. Previously, the deltaparent for revision r
was hardcoded to be r - 1.
The base revision of the delta chain can still be accessed as before, since it
is now computed in an iterative fashion, following the deltaparents backwards.