Check the exit code while invoking git in the force push check.

Previously, we were calling out to `popen` without asserting on the returned
exit-code. Now we raise a `RuntimeError` if the exit code is non-zero.

lib/gitlab/checks/force_push.rb

@@ -8,8 +8,13 @@ module Gitlab
         if Gitlab::Git.blank_ref?(oldrev) || Gitlab::Git.blank_ref?(newrev)
           false
         else
-          missed_ref, _ = Gitlab::Git::RevList.new(oldrev, newrev, project: project, env: env).execute
-          missed_ref.present?
+          missed_ref, exit_status = Gitlab::Git::RevList.new(oldrev, newrev, project: project, env: env).execute
+
+          if exit_status == 0
+            missed_ref.present?
+          else
+            raise RuntimeError, "Got a non-zero exit code while calling out to `git rev-list` in the force-push check."
+          end
         end
       end
     end

spec/lib/gitlab/checks/force_push_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Checks::ChangeAccess, lib: true do
+  let(:project) { create(:project) }
+
+  context "exit code checking" do
+    it "does not raise a runtime error if the `popen` call to git returns a zero exit code" do
+      allow(Gitlab::Popen).to receive(:popen).and_return(['normal output', 0])
+
+      expect { Gitlab::Checks::ForcePush.force_push?(project, 'oldrev', 'newrev') }.not_to raise_error
+    end
+
+    it "raises a runtime error if the `popen` call to git returns a non-zero exit code" do
+      allow(Gitlab::Popen).to receive(:popen).and_return(['error', 1])
+
+      expect { Gitlab::Checks::ForcePush.force_push?(project, 'oldrev', 'newrev') }.to raise_error(RuntimeError)
+    end
+  end
+end