title: Using subtree-merge strategy, history is not merging
body: <p>I'm trying to use an external SVN repository as a subtree in my repository by 'subtree-merging' it in.  I believe that this should keep the history of the files in the library intact, but it's not working - the files from the library that are merged into a subtree in my master branch have no history but for the commit when I add them - here's a history to show what I mean, precisely what I'm going to get to this state follows.</p>
  
  <pre><code>lappy8086:YACYAML jamie$ git log --graph 
  * commit 0cc6c4e5061741e67d009f3375ce1d2bcd3ab540
  | Author: James Montgomerie
  | Date:   Thu May 17 12:04:43 2012 +0100
  | 
  |     Subtree-merge in libYAML (from a git-svn checkout).
  |  
  * commit b5af5af109d77f6adafebc3dcf5a4796a5035a2e
  Author: James Montgomerie
  Date:   Thu May 17 11:47:32 2012 +0100
  
  First commit, add .gitignore.
  </code></pre>
  
  <p>Here's what I'm doing to try to get this to work:</p>
  
  <pre><code># check out SVN repo
  git svn clone http://svn.pyyaml.org/libyaml/branches/stable libYAML
  
  # create my repo
  mkdir YACYAML
  cd YACYAML
  git init
  touch .gitignore
  git add .gitignore
  git commit -m "First commit, add .gitignore"
  
  # Fetch from git-svn repo I got earlier
  git remote add libyaml-svn ../libYAML/
  git fetch libyaml-svn
  git checkout -b libyaml-svn libyaml-svn/master
  
  # Switch back to master, and try to merge in subtree
  git checkout master
  git read-tree --prefix=libYAML/ -u libyaml-svn/master
  git commit -m "Merge in libYAML as subtree (from git-svn checkout of SVN repo)"
  </code></pre>
  
  <p>This 'works', but, as I said, when I look at my history I expect to see the full history from the libYAML repo, but I don't - it's as above.</p>