Restore "project.name" (previously "project_id") to .arcconfig for arcanist

Summary:
A couple of things actually still care about this:

  - The code that detects if we're running `arc` on `arc`, and then runs the `arc` we're running on instead of the `arc` we ran, checks for the project being "arcanist" (`reenter_if_this_is_arcanist_or_libphutil()`).
  - Some of the symbol stuff still relies on it.
  - Removing it was really more to prove it works (which it seems to) than to accomplish any concrete goals.

Test Plan: looked at the file I guess

Reviewers: btrahan, talshiri

Reviewed By: talshiri

CC: aran

Differential Revision: https://secure.phabricator.com/D8076

.arcconfig

@@ -1,4 +1,5 @@
 {
+  "project.name" : "arcanist",
   "phabricator.uri" : "https://secure.phabricator.com/",
   "lint.engine" : "PhutilLintEngine",
   "unit.engine" : "PhutilUnitTestEngine",