t / t4101 / diff.3-0on commit worktree: teach "add" to check out existing branches (f60a7b7)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,2 @@
   4 a
   5-c
   6-b
   7\ No newline at end of file
   8+b