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

rust-chg: send client-side environment variables to server This is also needed to run config validation. The "validate" request and its response handling will be implemented in the next batch. Differential Revision: https://phab.mercurial-scm.org/D8364

rust-chg: add helper to pack environment variables On my machine, "printenv | wc -c" says 3422. That's the only reason why the initial buffer capacity is set to 4kB. Differential Revision: https://phab.mercurial-scm.org/D8363

rust-chg: move set_current_dir() to Locator This is necessary to run config validation in proper environment. Differential Revision: https://phab.mercurial-scm.org/D8362

rust-chg: abort if server doesn't have required capabilities It's checked at Locator::connect() since we'll have to do more stuff in this function. Differential Revision: https://phab.mercurial-scm.org/D8361

rust-chg: spawn server process if not running This is the minimal reimplementation of gethgcmd(), execcmdserver(), retryconnectcmdserver(), and connectcmdserver() in chg.c. No config validation is implemented yet. And some Py3 workarounds would be missing as this is the code I wrote in 2018. Differential Revision: https://phab.mercurial-scm.org/D8360