sidedata: simply read added files from the `ChangingFiles` object

sidedata: add a `decode_files_sidedata` function Right now the function mostly gather existing code to build a consistent object. However having this function allow us prepare all user code independently from the actual side data format change (and associated encoding/decoding changes) Strictly speaking, we could not need to passe the sidedata explicitly since we have access to it though the `changelogrevision` object. However, the short term goal is to drop that first parameter to only pass the sidedata binaries.

changelog: add a `changes` property on `changelogrevision` For the sidedata storage we are moving toward "all in one" block containing the equivalent of a "ChangingFiles" instance. We do various refactoring beforehand to prepare the usage of theses new data in the code. Since the object use slots, the "property cache" tricks cannot be used, and we cache the value manually.

sidedata: rename `encode_copies_sidedata` to `encode_files_sidedata` We are storing more than copies information, so lets make it clear.

log: pass around --rev option by walkopts

log: parse --limit option by logcmdutil.parseopts()

log: pass around --follow/--follow-first options by walkopts

log: introduce struct that carries log traversal options I tried to refactor logcmdutil.getrevs() without using an options struct, but none of these attempts didn't work out. Since every stage of getrevs() needs various log command options (e.g. both matcher and revset query need file patterns), it isn't possible to cleanly split getrevs() into a command layer and a core logic. So, this patch introduces a named struct to carry command options in slightly abstracted way, which will be later used by "hg grep" and "hg churn". More fields will be added to the walkopt struct. Type hints aren't verified. I couldn't figure out how to teach pytype to load its own attr type stubs in place of our .thirdparty.attr. Conditional import didn't work. s/^from \.thirdparty // is the only way I found pytype could parse the @attr.ib decorator.