getsubset: use None to request uncompressed changegroup

bundle: use None when unpacking uncompressed changegroup Let's be more modern!

writebundle: use 'None' instead of 'UN' for the bundle2 case Let's be modern!

compression: use 'None' for no-compression This seems more idiomatic and clearer. We still support both None and 'UN' for now because no user are migrated.

changegroup: move all compressions utilities in util We'll reuse the compression for other things (next target bundle2), so let's make it more accessible and organised.

obsolete: clarify that 'successorssets' returns the latest successors We do not return the first successors we found, we returns the latest (non obsolete (mostly)) one following the obsolete link transitively. We update the documentation to make this clean.

obsolete: drop an empty line at the end of a docstring This empty line is useless, let's drop it.

traceback: allow providing a local support contact point The extensions blaming code is fine for casual users but pretty terrible for corporate environments that can deploy a large amount of extensions to unsuspecting users. Reports will likely blame a random "innocent" extension (in our case crecord) and the hint in the message will triggers endless debug attempts from the user. We introduce a "ui.supportcontact" option that allow such big company to redirect their users to their own support desk. This disables all extensions blaming and just point people to the local support in all cases.