- * or is_git_directory() will reject the directory. Any valid
- * value would do because this value will be ignored and
- * replaced at the next (real) checkout.
+ * or is_git_directory() will reject the directory. Moreover, HEAD
+ * in the new worktree must resolve to the same value as HEAD in
+ * the current tree since the command invoked to populate the new
+ * worktree will be handed the branch/ref specified by the user.
+ * For instance, if the user asks for the new worktree to be based
+ * at HEAD~5, then the resolved HEAD~5 in the new worktree must
+ * match the resolved HEAD~5 in the current tree in order to match
+ * the user's expectation.