title: Google repo tool (master branch) fails to sync from local mirror on Windows
body: <p>Recently master branch for Google repo tool has been updated to support Windows OS. It is clear from commit logs : <a href="https://gerrit.googlesource.com/git-repo/+log" rel="nofollow noreferrer">https://gerrit.googlesource.com/git-repo/+log</a></p>
  
  <p>I am able to run basic commands like repo init and repo sync using repo tool on Windows (which makes use of MinGW Git on Windows). In my work I need to create a local mirror of a repository and then use it. repo tool works fine to create mirror repository.</p>
  
  <p>Here are commands used to create mirror repository on local drive on Windows OS.</p>
  
  <pre><code>$ repo.cmd init -u &lt;URL&gt; -b &lt;branch_name&gt; -m &lt;manifest_file_name&gt; --mirror --no-repo-verify
  
  $ repo.cmd sync --no-tags
  </code></pre>
  
  <ul>
  <li>--no-repo-verify is used to forcefully use master branch of repo tool.</li>
  <li>The above commands create the mirror in local drive C:\git-repo\test\mirror-testing\mirror</li>
  </ul>
  
  <p>But when I refer the above mirror to sync in other drive then it shows error.</p>
  
  <pre><code>Command:
  $ repo.cmd init -u &lt;URL&gt; -b &lt;Branch_name&gt; -m &lt;manifest_file_name&gt; --reference="C:\git-repo\test\mirror-testing\mirror" --no-repo-verify
  
  Error:
  error: object directory C:/git-repo/test/mirror-testing/mirror/project/manifest.git/objects? does not exist; check .git/objects/info/alternates.
  </code></pre>
  
  <p>The above command creates alternates file in C:/git-repo/test/mirror-testing/mirror/project/manifest.git/objects/info/ directory and that file contains below path:</p>
  
  <pre><code>C:\git-repo\test\mirror-testing\mirror\project/manifest.git\objects
  </code></pre>
  
  <p>I have tried other formats to provide reference value:</p>
  
  <pre><code>Git format: "/C/git-repo/test/mirror-testing/mirror"
  Cygwin format: "/cygdrive/c/git-repo/test/mirror-testing/mirror"
  </code></pre>
  
  <p>Using above formats in reference does not create alternates file in objects directory.</p>
  
  <p>repo init time in all above cases simply suggests that mirror is not referred and init is done from network. </p>
  
  <p>When I repeat the same test with Cygwin Git + Google repo tool (stable branch) then I do not see any error and repo init time and sync time is very less compared to the mirror init and sync time, which simply suggests that mirror is referred.</p>
  
  <ul>
  <li><p>repo.cmd is a batch file wrapper to invoke repo tool with python.
  repo.cmd file contains single line</p>
  
  <p>@call python %~dp0\repo %*</p></li>
  <li><p>I am using Cygwin terminal in Windows 7 to run these commands.</p></li>
  <li><p>To be clear repo init and repo sync shows error while referring to mirror but it completes successfully by syncing from outside network. The issue is that mirror is not referred.</p></li>
  </ul>
  
  <p>Has anyone used Google repo on Windows for creating mirror and syncing in other drive using as reference?</p>