https://gitlab.com/gitlab-org/gitaly/-/merge_requests/3537 updated the
default branch to be `master` instead of `main`. This broke
`backfill_snippet_repositories_spec.rb` because it expected to create
commits in `master`, but the default branch was `main`.

Since `main` is now the default, we should target the initial commit for
that branch.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/336538