"clean" means that index and work tree coincide, and "exists"/"nothing"
refer to the presence of a path in the specified commit:
+....
I H M Result
-------------------------------------------------------
0 nothing nothing nothing (does not happen)
19 no no yes exists exists keep index
20 yes yes no exists exists use M
21 no yes no exists exists fail
+....
In all "keep index" cases, the index entry stays as in the
original index file. If the entry is not up to date,