The revlog class does many things, among which fulfilling requests for
arbitrary byte slices from the revlog "data file" by reading a larger chunk
and caching it in memory, in order to reduce the number of system calls.
This extracts that logic into a new class, so that it may later also be used
for the side-data file (with another instance of that class).
The copyright notice of the new file does not include a date or author name
since such information tend not to be kept up-to-date: