title: Go Modules "unknown revision" error when using commit hash
body: <p>I need to pull <a href="https://github.com/libp2p/go-libp2p-core/commit/aca080dccfc2" rel="nofollow noreferrer">this</a> commit into my go project.</p>
  
  <p>I've tried multiple versions of <code>go.mod</code>:</p>
  
  <pre><code>...
  
  require (
      github.com/libp2p/go-libp2p-core@v0.0.7-0.20190626134135-aca080dccfc2
  
      // and...
      github.com/libp2p/go-libp2p-core v0.0.0-20190626-aca080dccfc2c9933df66baafe6cf9cc4f429825
  )
  
  ...
  </code></pre>
  
  <p>Both result in errors when running <code>$ go build</code>:</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ go build
  go: finding github.com/libp2p/go-libp2p-core v0.0.0-20190626-aca080dccfc2c9933df66baafe6cf9cc4f429825
  go: finding github.com/libp2p/go-libp2p-core v0.0.7-0.20190626134135-aca080dccfc2
  go: github.com/libp2p/go-libp2p-core@v0.0.0-20190626-aca080dccfc2c9933df66baafe6cf9cc4f429825: unknown revision v0.0.0-20190626-aca080dccfc2c9933df66baafe6cf9cc4f429825
  go: github.com/libp2p/go-libp2p-core@v0.0.7-0.20190626134135-aca080dccfc2: unknown revision aca080dccfc2
  go: error loading module requirements
  </code></pre>
  
  <p>Go getting doesn't work, either:</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ go get github.com/libp2p/go-libp2p-core@aca080dccfc2c9933df66baafe6cf9cc4f429825
  go: finding github.com/libp2p/go-libp2p-core v0.0.7-0.20190626134135-aca080dccfc2                                                                                                    go: github.com/libp2p/go-libp2p-core@v0.0.7-0.20190626134135-aca080dccfc2: unknown revision aca080dccfc2
  go: error loading module requirements
  </code></pre>
  
  <p>As @JimB points out, that hash wasn't merged and was rebased. So I tried to replace it with a new one, but it's still trying to fetch the old one?</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ go get github.com/libp2p/go-libp2p-core@v0.2.1-0.20190802003216-436d707f7cd0
  go: finding github.com/libp2p/go-libp2p-core v0.0.7-0.20190626134135-aca080dccfc2
  go: github.com/libp2p/go-libp2p-core@v0.0.7-0.20190626134135-aca080dccfc2: unknown revision aca080dccfc2
  go: error loading module requirements
  </code></pre>