Use set_reference over a write to .reference

`.reference` is a property whose setter function is `set_reference`.
My isn't happy when we try to assign directly:

```
scripts-dev/release.py:208: error: Trying to assign name "reference" that is not in "__slots__" of type "git.refs.head.HEAD"  [misc]
scripts-dev/release.py:208: error: Incompatible types in assignment (expression has type "HEAD", variable has type "Union[Head, TagReference, RemoteReference, Reference]")  [assignment]
```

but calling `set_reference` ourselves makes the typechecker happy.

Confusingly the following three types are all different:

- the `refererence` property: `Union['Head', 'TagReference', 'RemoteReference', 'Reference']`
- return type of `_get_reference`: `SymbolicReference`
- `set_reference` first argument: `Union[Commit_ish, 'SymbolicReference', str]`
  where `Commit_ish = Union['Commit', 'TagObject', 'Blob', 'Tree']`

It seems that typecheckers haven't settled down on The Way to handle
properties whose getter and setter types differ: see e.g.
python/mypy#3004 .

Author: David Robertson

scripts-dev/release.py

@@ -202,7 +202,7 @@ def prepare() -> None:
             click.get_current_context().abort()
 
         # Check out the base branch and ensure it's up to date
-        repo.head.reference = base_branch
+        repo.head.set_reference(base_branch, "check out the base branch")
         repo.head.reset(index=True, working_tree=True)
         if not base_branch.is_remote():
             update_branch(repo)