Add integration test for gemfile config option

Since this is likely going to be an important feature, add an
integration test that ensures it is working as intended.

The main reason this is difficult is because we are already running the
tests within a Bundler context, so we need to construct an artificial
environment that still exposes Overcommit as a gem without having to
have it installed ahead of time.

Author: sds

spec/integration/gemfile_option_spec.rb

@@ -0,0 +1,91 @@
+require 'spec_helper'
+
+describe 'specifying `gemfile` option in Overcommit configuration' do
+  let(:repo_root) { File.expand_path(File.join('..', '..'), File.dirname(__FILE__)) }
+  let(:fake_gem_path) { File.join('lib', 'my_fake_gem') }
+
+  # We point the overcommit gem back to this repo since we can't assume the gem
+  # has already been installed in a test environment
+  let(:gemfile) { normalize_indent(<<-RUBY) }
+    gem 'overcommit', path: '#{repo_root}'
+    gem 'my_fake_gem', path: '#{fake_gem_path}'
+  RUBY
+
+  let(:gemspec) { normalize_indent(<<-RUBY) }
+    Gem::Specification.new do |s|
+      s.name = 'my_fake_gem'
+      s.version = '1.0.0'
+      s.author = 'John Doe'
+      s.email = 'john.doe@example.com'
+      s.summary = 'A fake gem'
+      s.files = [File.join('lib', 'my_fake_gem.rb')]
+    end
+  RUBY
+
+  # Specify a hook that depends on an external gem to test Gemfile loading
+  let(:hook) { normalize_indent(<<-RUBY) }
+    module Overcommit::Hook::PreCommit
+      class FakeHook < Base
+        def run
+          require 'my_fake_gem'
+          :pass
+        end
+      end
+    end
+  RUBY
+
+  let(:config) { normalize_indent(<<-YAML) }
+    verify_plugin_signatures: false
+
+    CommitMsg:
+      ALL:
+        enabled: false
+
+    PreCommit:
+      ALL:
+        enabled: false
+      FakeHook:
+        enabled: true
+        requires_files: false
+  YAML
+
+  around do |example|
+    repo do
+      # Since RSpec is being run within a Bundler context we need to clear it
+      # in order to not taint the test
+      Bundler.with_clean_env do
+        FileUtils.mkdir_p(File.join(fake_gem_path, 'lib'))
+        echo(gemspec, File.join(fake_gem_path, 'my_fake_gem.gemspec'))
+        FileUtils.touch(File.join(fake_gem_path, 'lib', 'my_fake_gem.rb'))
+
+        echo(gemfile, '.overcommit_gems.rb')
+        `bundle install --gemfile=.overcommit_gems.rb`
+
+        echo(config, '.overcommit.yml')
+
+        # Set BUNDLE_GEMFILE so we load Overcommit from the current repo
+        `BUNDLE_GEMFILE=.overcommit_gems.rb bundle exec overcommit --install > #{File::NULL}`
+        FileUtils.mkdir_p(File.join('.git-hooks', 'pre_commit'))
+        echo(hook, File.join('.git-hooks', 'pre_commit', 'fake_hook.rb'))
+
+        example.run
+      end
+    end
+  end
+
+  subject { shell(%w[git commit --allow-empty -m Test]) }
+
+  context 'when configuration specifies the gemfile' do
+    let(:config) { "gemfile: .overcommit_gems.rb\n" + super() }
+
+    it 'runs the hook successfully' do
+      subject.status.should == 0
+    end
+  end
+
+  context 'when configuration does not specify the gemfile' do
+    it 'fails to run the hook' do
+      subject.status.should_not == 0
+    end
+  end
+end

spec/support/normalize_indent.rb

@@ -0,0 +1,12 @@
+# Strips off excess leading indentation from each line so we can use Heredocs
+# for writing code without having the leading indentation count.
+module IndentNormalizer
+  def normalize_indent(code)
+    leading_indent = code[/^(\s*)/, 1]
+    code.lstrip.gsub(/\n#{leading_indent}/, "\n")
+  end
+end
+
+RSpec.configure do |_config|
+  include IndentNormalizer
+end