Ensure that catalog is initialized with branch master

Before this commit, Commodore could fail to push the initial catalog for
a cluster if the user's Git config sets `init.defaultBranch`. The
observed error is:

```
 > Commiting changes...
 > Pushing catalog to remote...
Error: Failed to push to the catalog repository: Git exited with status code 1
The error reported was:
  stderr: 'error: src refspec master does not match any
error: failed to push some refs to 'ssh://<catalog repo>''
```

This error is caused because GitPython falls back to `init.defaultBranch`
when creating the catalog repo unless `initial_branch` is specified.

However, since Commodore unconditionally falls back to trying to push
`master` when no default branch can be identified in the remote repo
(e.g. empty catalog repo), the push then fails because we're trying to
push branch `master` which doesn't exist locally.

This isn't an issue for `commodore component new` and `commodore package
new` since we explicitly fall back to creating a `master` branch when
initializing the worktree for the new dependency.

commodore/gitrepo/__init__.py

@@ -115,7 +115,7 @@ def __init__(
         if not force_init and targetdir.exists():
             self._repo = Repo(targetdir)
         else:
-            self._repo = Repo.init(targetdir, bare=bare)
+            self._repo = Repo.init(targetdir, bare=bare, initial_branch="master")
 
         if remote:
             self.remote = remote