Renaming a category to the name of an existing category is not refused

When categories are stored in Blog.Categories (the default since 9.15) or in the categories space of a custom blog, renaming a category to the name of another existing category of the same blog is not refused.

Steps to reproduce

1. Have two categories Blog.Categories.A and Blog.Categories.B.
2. In Blog.ManageCategories, rename A to "B".

Expected

An error saying that the category B already exists.

Actual

The "already exists" check is done on Blog.B (the new name resolved relative to Blog.ManageCategories), while the rename targets Blog.Categories.B. The check passes and the rename is attempted over the existing category.

The rename target is computed in #renameCategory (Blog.CategoriesCode) from the space of the renamed category, while the checks in Blog.ManageCategories use the raw new name.