Automatically set up arcrc if it's missing

Summary:
Right now the instructions tell you to do it manually.  But we can just
do it for you!  (Well, you'll have to do what it says, but at least you
don't have to be instructed to run the command.)

Depends on D45131 for this command to be sufficient.

Test Plan:
`rm ~/.arcrc /tmp/arc.pull` then `./setup.sh` should set up a
new arcrc (with my help); running again should no-op.

Reviewers: csilvers, jacqueline

Reviewed By: csilvers

Subscribers: #devops

Differential Revision: https://phabricator.khanacademy.org/D45132

setup.sh

@@ -330,6 +330,24 @@ install_hooks() {
     fi
 }
 
+# Set up .arcrc: we can't update this through the standard process
+# because it has secrets embedded in it, but our arcanist fork will
+# do the updates for us.
+setup_arc() {
+    if [ ! -f "$HOME/.arcrc" ]; then
+        echo "Time to set up arc to talk to Phabricator!"
+        echo "First, go make sure you're logged in and your"
+        echo "account is set up (use Google OAuth to create"
+        echo "an account, if you haven't).  Click here to start:"
+        echo "  -->  https://phabricator.khanacademy.org  <--"
+        echo -n "Press enter when you're logged in: "
+        read
+        # This is added to PATh by dotfiles, but those may not be sourced yet.
+        PATH="$DEVTOOLS_DIR/arcanist/khan-bin:$PATH"
+        arc install-certificate https://phabricator.khanacademy.org
+    fi
+}
+
 
 check_dependencies
 
@@ -344,9 +362,10 @@ edit_system_config
 # the order for these is (mostly!) important, beware
 clone_repos
 install_and_setup_gcloud
-install_deps        # pre-req: clone_repos, install_and_setup_gcloud
+install_deps        # pre-reqs: clone_repos, install_and_setup_gcloud
 install_hooks       # pre-req: clone_repos
-download_db_dump    # pre-reqs: install_deps
+setup_arc           # pre-req: clone_repos
+download_db_dump    # pre-req: install_deps
 
 
 echo