Windows: Ensure that merges that containing symlinks do not corrupt symlinks.

#297 is open for full support of symlinks on Windows. Until that's ready, however, we should ensure that merging two branches where a symlink has changed works properly on Windows and does not corrupt the symlink.

Example

1. On Mac: Create a branch in a repo containing a symlink
2. Modify the symlink, commit the change, and push it to the server
3. On Windows: Fetch the new branch
4. Merge in the new branch and push it to the server
5. On Mac: Fetch the (now merged) branch
6. Confirm that the symlink is still a symlink

We should also test how Windows handles merge conflicts of symlinks (although adding support for the symlink merge conflict scenario likely falls under #297 rather than this issue).