revlog: add clone method Upcoming patches will introduce functionality for in-place repository/store "upgrades." Copying the contents of a revlog feels sufficiently low-level to warrant being in the revlog class. So this commit implements that functionality. Because full delta recomputation can be *very* expensive (we're talking several hours on the Firefox repository), we support multiple modes of execution with regards to delta (re)use. This will allow repository upgrades to choose the "level" of processing/optimization they wish to perform when converting revlogs. It's not obvious from this commit, but "addrevisioncb" will be used for progress reporting.