0 nothing nothing nothing (does not happen)
1 nothing nothing exists use M
2 nothing exists nothing remove path from index
- 3 nothing exists exists use M
+ 3 nothing exists exists, use M if "initial checkout"
+ H == M keep index otherwise
+ exists fail
+ H != M
clean I==H I==M
------------------
merge, but it would not show in `git diff-index --cached $M`
output after two-tree merge.
+Case #3 is slightly tricky and needs explanation. The result from this
+rule logically should be to remove the path if the user staged the removal
+of the path and then swiching to a new branch. That however will prevent
+the initial checkout from happening, so the rule is modified to use M (new
+tree) only when the contents of the index is empty. Otherwise the removal
+of the path is kept as long as $H and $M are the same.
3-Way Merge
~~~~~~~~~~~