Check that we are not overwriting a local repository when syncing.

If a local git repository exists within the same folder as a new project that
is added, when the user syncs the repo, the sync will overwrite the local
files under the project's .git repository with its own symlinks. Make sure
that we do not overwrite 'normal' files in repo and throw an error when
that happens.

project.py

@@ -1120,7 +1120,10 @@ class Project(object):
         try:
           src = os.path.join(self.gitdir, name)
           dst = os.path.join(dotgit, name)
-          os.symlink(relpath(src, dst), dst)
+          if os.path.islink(dst) or not os.path.exists(dst):
+            os.symlink(relpath(src, dst), dst)
+          else:
+            raise GitError('cannot overwrite a local work tree')
         except OSError, e:
           if e.errno == errno.EPERM:
             raise GitError('filesystem must support symlinks')