R1:1c1f749eba21

Add an "arcanist.projectinfo" Conduit call

Summary:
We currently rely on "remote_hooks_enabled" in .arcconfig to determine whether
commands like "arc amend" and "arc merge" should imply "arc mark-committed".

However, this is a historical artifact that is now bad for a bunch of reasons:

- The option name is confusing, it really means 'repository is tracked'.
- The option is hard to discover and generally sucks.
- We can empirically determine the right answer since we now know if a project
is in a…