title: Capistrano fails authenticate with git
body: <p>While trying to deploy a website using capistrano, I get the following message:</p>
  
  <pre><code>  INFO ---------------------------------------------------------------------------
    INFO START 2016-10-19 21:56:37 +0200 cap staging deploy
    INFO ---------------------------------------------------------------------------
    INFO [95790a6d] Running /usr/bin/env mkdir -p /tmp as mysite@staging.mysite.com
   DEBUG [95790a6d] Command: /usr/bin/env mkdir -p /tmp
    INFO [95790a6d] Finished in 0.407 seconds with exit status 0 (successful).
   DEBUG Uploading /tmp/git-ssh-mysite-staging-username.sh 0.0%
    INFO Uploading /tmp/git-ssh-mysite-staging-username.sh 100.0%
    INFO [0cd82d0b] Running /usr/bin/env chmod 700 /tmp/git-ssh-mysite-staging-username.sh as mysite@staging.mysite.com
   DEBUG [0cd82d0b] Command: /usr/bin/env chmod 700 /tmp/git-ssh-mysite-staging-username.sh
    INFO [0cd82d0b] Finished in 0.029 seconds with exit status 0 (successful).
    INFO [0265a2c7] Running /usr/bin/env git ls-remote --heads git@bitbucket.org:username/mysite.git as mysite@staging.mysite.com
   DEBUG [0265a2c7] Command: ( export GIT_ASKPASS="/bin/echo" GIT_SSH="/tmp/git-ssh-mysite-staging-username.sh" ; /usr/bin/env git ls-remote --heads git@bitbucket.org:username/mysite.git )
   DEBUG [0265a2c7]       conq: repository access denied.
   DEBUG [0265a2c7]       fatal: Could not read from remote repository.
  
  Please make sure you have the correct access rights
  and the repository exists.
  </code></pre>
  
  <p>However when I run this command manually, as the same user, everything works fine:</p>
  
  <pre><code>$ export GIT_ASKPASS="/bin/echo" GIT_SSH="/tmp/git-ssh-mysite-staging-username.sh" ; /usr/bin/env git ls-remote --heads git@bitbucket.org:username/mysite.git
  928c1e393778f25869f14958151c4ea1dc7d68b6    refs/heads/master
  </code></pre>
  
  <p>I've verified that my public key is bitbucket deployment keys for the repository. This worked fine before. Is there any way I can debug this further?</p>