title: alembic revision - multiple heads (due branching) error
body: <p>I've got an application and I wanted to create a new migration for it today. When I run </p>
  
  <pre><code>$ alembic revision -m "__name__"
  </code></pre>
  
  <p>I got a message</p>
  
  <pre><code>Only a single head is supported. The script directory has multiple heads (due branching), which must be resolved by manually editing the revision files to form a linear sequence. 
  Run `alembic branches` to see the divergence(s).
  </code></pre>
  
  <p>Running </p>
  
  <pre><code>alembic branches
  </code></pre>
  
  <p>gives nothing </p>
  
  <p>I'm new to alembic. There are 2 developers working on this app and we have 2 git branches - master &amp; develop (I'm not sure if this have anything to do with it).</p>
  
  <p>Any clue on what is this about?</p>