rust-chg: add config validation and process returned instructions This is the reimplementation of runinstructions() and main() in chg.c. In Rust version, we only pass in early arguments to the server as the locator doesn't know the full arguments. This should be fine since these arguments are just passed in to _earlyparseopts() and _parseconfig(), which means the server doesn't need full arguments. Another difference is the handling of the "exit <code>" instruction. In Rust version, we can simply reuse the connection instead of "exit(code)" as the command error isn't displayed yet. That's because the client-side stdio is not attached until the connection is validated. This behavior is cleaner than C, but it also means that the early server exception wouldn't be propagated to client because stderr isn't attached. So we might have to reconsider when to attach/detach the server stdio. Differential Revision: https://phab.mercurial-scm.org/D8381

rust-chg: collect server flags from command arguments This is the reimplementation of testsensitiveflag() and setcmdserverargs() of chg.c. Differential Revision: https://phab.mercurial-scm.org/D8380

rust-chg: add interface to run "validate" request Differential Revision: https://phab.mercurial-scm.org/D8379

rust-chg: add helper to parse instructions sent from server This is well structured version of runinstructions() of chg.c. Differential Revision: https://phab.mercurial-scm.org/D8378

tests: move verification closer to setup in test-copies-chain-merge.t I found this test case really hard to read because it requires scrolling back and forth between the setup and the verification. This patch moves them closer together. There was also duplicated descriptions for each test case (duplicated between setup and verification). They had already become out of sync. I picked the description from the verification, partly so the differences would be highlighted in this patch. Differential Revision: https://phab.mercurial-scm.org/D8377

tests: collect all branch creation in one place in test-copies-chain-merge.t I found this test case really hard to read because it requires scrolling back and forth between the setup and the verification. The next patch will move the verification close to the merge commits they test. This patch prepares for that by moving all branch creation first so they are separate from the merge commits (because many merge commits reuse the same branches). Differential Revision: https://phab.mercurial-scm.org/D8376

hgk: remove a "b" used on a kwargs expansion, the keys are strs Differential Revision: https://phab.mercurial-scm.org/D8385