title: get diff between files in two different git repos and apply to first repo
body: <p>I have two different local git repos, which we'll call <code>a</code> and <code>b</code>. They are both in a <code>projects</code> folder in my home directory. I want to get a diff between a particular file in both repos, let's call it <code>foo</code>, and apply that diff to <code>a</code>.</p>
  
  <p>if I run <code>git diff ./foo ../b/foo &gt; diff.diff</code> from inside <code>a</code> I can look at the diff and confirm that it contains the changes I want to apply, but when I run <code>git apply diff.diff</code> then I get this error:</p>
  
  <pre><code>fatal: invalid path './foo' 
  </code></pre>
  
  <p>If I try running <code>git diff ~/projects/a/foo ~/projects/b/foo &gt; diff.diff</code>, then when I try to apply the diff I get this error: </p>
  
  <pre><code>error: home/ryan/projects/a/foo: No such file or directory
  </code></pre>
  
  <p>Now, I can just copy the file to get the same effect as applying the diff, but is there a good reason why this isn't working?</p>