Fix error when using --unreleased flag

Fixes #16

Author: cookpete

src/releases.js

@@ -14,7 +14,7 @@ export function parseReleases (commits, origin, packageVersion, options) {
           ...release,
           href: getCompareLink(commit.tag, release.tag || 'HEAD', origin),
           commits: release.commits.sort(sortCommits),
-          major: semver.diff(commit.tag, release.tag) === 'major'
+          major: commit.tag && release.tag && semver.diff(commit.tag, release.tag) === 'major'
         })
       }
       release = newRelease(commit.tag, commit.date)

test/run.js

@@ -56,6 +56,13 @@ describe('run', () => {
     })
   })
 
+  it('supports unreleased option', () => {
+    return run(['', '', '--unreleased']).then(message => {
+      expect(message).to.be.a('string')
+      expect(message).to.have.string('bytes written to')
+    })
+  })
+
   it('uses options from package.json', async () => {
     const expected = await readFile(join(__dirname, 'data', 'template-keepachangelog.md'), 'utf-8')