revlog: change accesses to index entry elements to use positive offsets

revlog: parse revlogv0 indexes into v1 internally This lets us eliminate lots of special case code in our performance-sensitive index accessors.

revlog: only allow lazy parsing with revlogng files This will allow us to store indices in memory in a single entry format

revlog: simplify the v1 immediate parser - read all the data at once (large files are handled by the lazy parser) - cache the entire file for inline revlogs - simplify looping

revlog: set the threshold for lazy parsing higher A typical machine can parse a 1MB index in well under a second

revlog: simplify the v0 parser

revlog: add revlogio interface This lets us separate the logic for different on-disk revlog formats from the main revlog logic.

revlog: regroup parsing code