Use a shared example for changes and exists glob matching rules

This ensures that the glob matching used by all rules are
consistent.

spec/lib/gitlab/ci/build/rules/rule/clause/changes_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Ci::Build::Rules::Rule::Clause::Changes do
+  describe 'satisfied_by?' do
+    let(:pipeline) { build(:ci_pipeline) }
+    subject { described_class.new(globs) }
+
+    before do
+      allow(pipeline).to receive(:modified_paths).and_return(files.keys)
+    end
+
+    it_behaves_like 'a glob matching rule'
+  end
+end

spec/lib/gitlab/ci/build/rules/rule/clause/exists_spec.rb

@@ -4,27 +4,10 @@ require 'spec_helper'
 
 describe Gitlab::Ci::Build::Rules::Rule::Clause::Exists do
   describe 'satisfied_by?' do
-    using RSpec::Parameterized::TableSyntax
+    let(:project) { create(:project, :custom_repo, files: files) }
+    let(:pipeline) { build(:ci_pipeline, project: project, sha: project.repository.head_commit.sha) }
+    subject { described_class.new(globs) }
 
-    where(:case_name, :globs, :files, :satisfied) do
-      'exact top-level match'      | ['Dockerfile']               | { 'Dockerfile' => '', 'Gemfile' => '' }           | true
-      'exact top-level no match'   | ['Dockerfile']               | { 'Gemfile' => '' }                               | false
-      'pattern top-level match'    | ['Docker*']                  | { 'Dockerfile' => '', 'Gemfile' => '' }           | true
-      'pattern top-level no match' | ['Docker*']                  | { 'Gemfile' => '' }                               | false
-      'exact nested match'         | ['project/build.properties'] | { 'project/build.properties' => '' }              | true
-      'exact nested no match'      | ['project/build.properties'] | { 'project/README.md' => '' }                     | false
-      'pattern nested match'       | ['src/**/*.go']              | { 'src/gitlab.com/goproject/goproject.go' => '' } | true
-      'pattern nested no match'    | ['src/**/*.go']              | { 'src/gitlab.com/goproject/README.md' => '' }    | false
-    end
-
-    with_them do
-      let(:project) { create(:project, :custom_repo, files: files) }
-      let(:pipeline) { build(:ci_pipeline, project: project, sha: project.repository.head_commit.sha) }
-      subject { described_class.new(globs) }
-
-      it 'checks if any files exist' do
-        expect(subject.satisfied_by?(pipeline, nil)).to eq(satisfied)
-      end
-    end
+    it_behaves_like 'a glob matching rule'
   end
 end

spec/support/shared_examples/lib/gitlab/ci/build/rules/rule/clause/clause_shared_examples.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'a glob matching rule' do
+  using RSpec::Parameterized::TableSyntax
+
+  where(:case_name, :globs, :files, :satisfied) do
+    'exact top-level match'      | ['Dockerfile']               | { 'Dockerfile' => '', 'Gemfile' => '' }           | true
+    'exact top-level no match'   | ['Dockerfile']               | { 'Gemfile' => '' }                               | false
+    'pattern top-level match'    | ['Docker*']                  | { 'Dockerfile' => '', 'Gemfile' => '' }           | true
+    'pattern top-level no match' | ['Docker*']                  | { 'Gemfile' => '' }                               | false
+    'exact nested match'         | ['project/build.properties'] | { 'project/build.properties' => '' }              | true
+    'exact nested no match'      | ['project/build.properties'] | { 'project/README.md' => '' }                     | false
+    'pattern nested match'       | ['src/**/*.go']              | { 'src/gitlab.com/goproject/goproject.go' => '' } | true
+    'pattern nested no match'    | ['src/**/*.go']              | { 'src/gitlab.com/goproject/README.md' => '' }    | false
+    'leading slash no match'     | ['/*.go']                    | { 'main.go' => '' }                               | false
+  end
+
+  with_them do
+    it 'checks if any files exist' do
+      expect(subject.satisfied_by?(pipeline, nil)).to eq(satisfied)
+    end
+  end
+end