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.
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