Base content title on ContentBaseName (#440)

With Hugo v0.88.1 and earlier, ContentBaseName and TranslationBaseName
both returned "index" when creating a leaf bundle with hugo new. This is
a known bug, addressed with https://github.com/gohugoio/hugo/pull/9045.

With this PR:

  hugo new post/my-first-post/index.md

will produce:

  title: "My First Post"

instead of:

  title: "Index"