test: add test to prevent duplicate headers

Add a failing test that proves conventional-changelog#305 so that a subsequent commit can fix
it.

test.js

@@ -151,7 +151,7 @@ describe('cli', function () {
   })
 
   describe('CHANGELOG.md exists', function () {
-    it('appends the new release above the last release, removing the old header', function () {
+    it('appends the new release above the last release, removing the old header (legacy format)', function () {
       fs.writeFileSync('CHANGELOG.md', 'legacy header format<a name="1.0.0">\n', 'utf-8')
 
       commit('feat: first commit')
@@ -164,6 +164,25 @@ describe('cli', function () {
       content.should.not.match(/legacy header format/)
     })
 
+    it('appends the new release above the last release, removing the old header (new format)', function () {
+      commit('feat: first commit')
+      shell.exec('git tag -a v1.0.0 -m "my awesome first release"')
+      commit('fix: patch release')
+
+      execCli().code.should.equal(0)
+      var content = fs.readFileSync('CHANGELOG.md', 'utf-8')
+
+      // remove commit hashes and dates to make testing against a static string easier:
+      content = content.replace(/patch release [0-9a-f]{6,8}/g, 'patch release ABCDEFXY').replace(/\([0-9]{4}-[0-9]{2}-[0-9]{2}\)/g, '(YYYY-MM-DD)')
+      content.should.equal('# Change Log\n\nAll notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.\n\n## [1.0.1](/compare/v1.0.0...v1.0.1) (YYYY-MM-DD)\n\n\n### Bug Fixes\n\n* patch release ABCDEFXY\n')
+
+      commit('fix: another patch release')
+      execCli().code.should.equal(0)
+      content = fs.readFileSync('CHANGELOG.md', 'utf-8')
+      content = content.replace(/patch release [0-9a-f]{6,8}/g, 'patch release ABCDEFXY').replace(/\([0-9]{4}-[0-9]{2}-[0-9]{2}\)/g, '(YYYY-MM-DD)')
+      content.should.equal('# Change Log\n\nAll notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.\n\n## [1.0.2](/compare/v1.0.1...v1.0.2) (YYYY-MM-DD)\n\n\n### Bug Fixes\n\n* another patch release ABCDEFXY\n\n\n\n## [1.0.1](/compare/v1.0.0...v1.0.1) (YYYY-MM-DD)\n\n\n### Bug Fixes\n\n* patch release ABCDEFXY\n')
+    })
+
     it('commits all staged files', function () {
       fs.writeFileSync('CHANGELOG.md', 'legacy header format<a name="1.0.0">\n', 'utf-8')