Initialize git in copied revisions to resolve setuptools-scm issues.

Fixes #26.

The following error message arises when using `setuptools-scm`:

```
LookupError: setuptools-scm was unable to detect version for ...

      Make sure you're either building from a fully intact git repository or PyPI tarballs. Most other sources (such as GitHub's tarballs, a git checkout without the .git folder) don't contain the necessary metadata and will not work.

      For example, if you're using pip, instead of https://github.com/user/proj/archive/master.zip use git+https://github.com/user/proj.git#egg=proj

      Alternatively, set the version with the environment variable SETUPTOOLS_SCM_PRETEND_VERSION_FOR_${NORMALIZED_DIST_NAME} as described in https://setuptools-scm.readthedocs.io/en/latest/config.
      [end of output]
```

This can be resolved by creating an empty git respository.

Author: vpratz

sphinx_polyversion/git.py

@@ -202,6 +202,7 @@ async def _copy_tree(
     """
     # retrieve commit contents as tar archive
     cmd = ("git", "archive", "--format", "tar", ref)
+    git_init_cmd = ("git", "init", "--initial-branch=dummy")
     with tempfile.SpooledTemporaryFile(max_size=buffer_size) as f:
         process = await asyncio.create_subprocess_exec(
             *cmd, cwd=repo, stdout=f, stderr=PIPE
@@ -213,6 +214,13 @@ async def _copy_tree(
         f.seek(0)
         with tarfile.open(fileobj=f) as tf:
             tf.extractall(str(dest))
+        # initialize dummy git repository in copied directory (required for setuptools-scm)
+        process = await asyncio.create_subprocess_exec(
+            *git_init_cmd, cwd=str(dest), stdout=f, stderr=PIPE
+        )
+        out, err = await process.communicate()
+        if process.returncode:
+            raise CalledProcessError(process.returncode, " ".join(cmd), stderr=err)
 
 
 async def file_exists(repo: Path, ref: GitRef, file: PurePath) -> bool: