Mark Phabricator as having remote hooks installed

Summary:
The name of this config option is super confusing since it really just means "do
not implicitly 'mark-committed' when running 'arc commit' or 'arc amend'" but
that's kind of a lot of text. Not sure what best to do about this.

If this looks ok, I'll also make this change to libphutil, diviner, arcanist and
javelin.

Test Plan:
will amend

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, aran, jungejason
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 245

.arcconfig

@@ -4,6 +4,7 @@
   "lint_engine" : "PhutilLintEngine",
   "unit_engine" : "PhutilUnitTestEngine",
   "copyright_holder" : "Facebook, Inc.",
+  "remote_hooks_installed" : true,
   "phutil_libraries" : {
     "phabricator" : "src/"
   }