+ Tree to checkout from (when paths are given). If not specified,
+ the index will be used.
- Branch to checkout; may be any object ID that resolves to a
- commit. Defaults to HEAD.
+ Branch to checkout (when no paths are given); may be any object
+ ID that resolves to a commit. Defaults to HEAD.
When this parameter names a non-branch (but still a valid commit object),
your HEAD becomes 'detached'.