debugdiscovery: move various computation earlier We are about to add more data to debug discovery (eg: data bout the initial undecided set, number of roundtrip, etc). So we start by cleaning up the code by spliting some computation and some display related preparation. Differential Revision: https://phab.mercurial-scm.org/D9524

debugdiscovery: clarify internal key name in debugobsolete They were probably clear when they got added initially but with more key around, they gain to be clearer. Differential Revision: https://phab.mercurial-scm.org/D9528

copies-rust: extract the processing of a ChangedFiles in its own function This is a reasonably independent piece of code that we can extract in its own function. This extraction will be very useful for the next changeset, where we will change the iteration order (but still do the same kind of processing). Differential Revision: https://phab.mercurial-scm.org/D9421

copies-rust: move the parent token to an enum We carry around information about which parent of a revision is been dealt with. So far this was a `usize` but as we are about to pass it around to more function it seems like a good idea to start cleaning this up and use a proper enum. Differential Revision: https://phab.mercurial-scm.org/D9420