title: Maven release:prepare fails to commit tag
body: <p>Attempting to perform a release on a maven project, which has successfully released before.</p>
  
  <p>When I perform <code>mvn release:prepare</code> I am prompted for the release tags and the new snapshot tags and the project builds.</p>
  
  <p>But when it attempts to push to the remote, I get</p>
  
  <p><code>
      [ERROR] Failed to execute goal org.apache.maven.plugins:maven-release-plugin:2.0:prepare (default-cli) on project NeuralAnalysis: Unable to tag SCM
      [ERROR] Provider message:
      [ERROR] The git-push command failed.
      [ERROR] Command output:
      [ERROR] To ssh://gitosis@10.41.129.2/NeuralAnalysis.git
      [ERROR] ! [rejected]        NeuralAnalysis-1.5.6 -> NeuralAnalysis-1.5.6 (non-fast-forward)
      [ERROR] error: failed to push some refs to 'ssh://gitosis@10.41.129.2/NeuralAnalysis.git'
      [ERROR] To prevent you from losing history, non-fast-forward updates were rejected
  </code></p>
  
  <p>And indeed, attempting to perform <code> git push ssh://gitosis@10.41.129.2/NeuralAnalysis.git NeuralAnalysis-1.5.6</code> manually, also exits with the same complaint. </p>
  
  <p>Performing <code>git pull</code> says 'Already up-to-date'. <code>git branch</code> shows I am on the 'master'. <code>git push origin</code> gives 'Everything up-to-date'. </p>
  
  <p>Looking at the repository using Tower shows that 'master', 'origin/master' and 'NeuralAnalysis-1.5.6' are all the same and on the last commit. The working directory contains release.properties and pom.xml.releaseBackup files.</p>
  
  <p>It looks to me like everything is fine with the repository overall.</p>