title: Signing commit with OpenPGP subkey fails
body: <p>I would like to use one of my GPS (2) subkeys for signing commits/tags in Git
  I.e., my freshly created RSA4096 signing-only key with the long ID B0##...</p>
  
  <pre><code>sec#  ed25519/9F############## 2016-01-07 [expires: 2023-01-05]
    Key fingerprint = FC08 HEX HEX HEX 
  uid                 [ultimate] MY NAME &lt;MY.NAME@foo bar&gt;
  ssb   rsa4096/C9############## 2016-01-07 [expires: 2022-01-05]
  ssb   ed25519/C6############## 2016-01-07 [expires: 2022-01-05]
  ssb   rsa4096/B0############## 2016-01-13 [expires: 2022-01-11]
  </code></pre>
  
  <p>Where I am working on a keyring with the master key removed (backuped away) as 'better key policy'</p>
  
  <p>So, I tried to set up the signing key for Git</p>
  
  <pre><code>[user]
      ...
      signingkey = B0##############
  </code></pre>
  
  <p>However, committing &amp; signing fails with</p>
  
  <pre><code>&gt; git commit -S  -m "test commit"
  gpg: skipped "B0##############": secret key not available
  gpg: signing failed: secret key not available
  error: gpg failed to sign the data
  fatal: failed to write commit object
  </code></pre>
  
  <p>Where a gpg-agent up and running.</p>
  
  <p>My first guess was, that Git does not understand the long key notation and tried the short one instead</p>
  
  <pre><code>&gt; gpg2 --list-secret-keys  --keyid-format short
  ...
  ssb   rsa4096/DB###### 2016-01-13 [expires: 2022-01-11]
  
  &gt; ~/.gitconfig
  [user]
     ...
     signingkey = DB######
  </code></pre>
  
  <p>But which also failed</p>
  
  <pre><code>&gt; git commit -S  -m "test commit short"
  gpg: skipped "DB######": secret key not available
  gpg: signing failed: secret key not available
  error: gpg failed to sign the data
  fatal: failed to write commit object
  </code></pre>
  
  <p>So, I wonder what breaks here and if maybe Git only would work with a master key for signing but does not understand the use of subkeys (or if I have screwed up myself somewhere)?</p>