revlog: split constants into a new `revlogutils.constants` module
We want to split some logic out of the main revlog file (the delta computing
logic). However, this logic needs access to multiple constants related to the
revlog. So we move all revlog related constants into a new module that could
be imported from multiple places.
We don't copy the file (preserving blame history) because there are only a few
moving lines. Also, copying the file would result in annoying merge conflicts
with ongoing work from others contributors.
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