treemanifests: make _loadlazy tolerate item not on _lazydirs I'd like to clean up a few cases where we check for an item in _lazydirs before calling _loadlazy - this will remove an extraneous dict lookup and make it slightly more versatile. Differential Revision: https://phab.mercurial-scm.org/D4842

debugcommands: add a debugindexstats command Someone went through the trouble of recording these stats, so let's make them accessible. Differential Revision: https://phab.mercurial-scm.org/D4832

rust-chg: add main program It can at least connect to a *running* command server, and execute Mercurial command. No signal handling nor daemon management is implemented yet.

rust-chg: add interface to chdir the server

rust-chg: add Client extensions to run cHg-specific requests This just provides a nicer way to issue command-server requests.

rust-chg: port basic socket path handling from cHg of C This is basically modeled after setcmdserveropts() of chg.c.

rust-chg: add state machine to handle "runcommand" request with cHg extension This is modeled after tokio-hglib's RunCommand state to support the "S" channel message.

rust-chg: add callback to handle pager and shell command requests This could be inlined into the ChgRunCommand state to be introduced by the next patch, but it seemed good to separate any user interactions from the IPC code.

rust-chg: add low-level function to set pager fd blocking This is necessary because the server expects stdout/stderr to be blocking, whereas we'll use async library to spawn pager, which makes pipes unblocking.

rust-chg: add future that handles "attachio" request This is the sequence to send client-side stdio and pager stdin to the server.