Fixes the `Open Diff` action for untracked files when the `sort_by_path`
setting is enabled. The `ProjectDiff` wasn't correctly moving the
multibuffer's cursor to the untracked file because, when that setting is
enabled, it's sort prefix is changed to the tracked files sort prefix, and that
wasn't accounted for in `move_to_entry`.
Before these changes, the `sort_prefix` field for `PathKey` was called `namespace`, it was renamed to be clearer what its purpose is.
Closes#39529
Release Notes:
- Fixed 'Open Diff' action for untracked files when `sort_by_path` is
enabled
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Co-authored-by: David Kleingeld <davidsk@zed.dev>