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