`mg repl` is essentially a shortcut for nix repl $(mg path //) which
comes up often enough for me. Launching a repl only really makes sense
in the repository root with how readTree works at the moment, so I think
this is a convenient addition.
Change-Id: I32b695885c2e6eaecdcc656c7249afa504439913
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5822
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
89f3f0a5dc