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