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 tracked repository.

Add a call which arcanist can make on these workflows to figure out if it is
interacting with a project in a tracked repository or not.

Also added an "isTracked()" convenience method to reduce the number of magic
strings all over the place.

Test Plan: Ran "arcanist.projectinfo" for nonexistent, untracked and tracked
projects.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: Makinde

CC: aran, epriestley, Makinde

Differential Revision: 945