When things cleanly merge, these things happen:
-1. the results are updated both in the index file and in your
- working tree,
-2. index file is written out as a tree,
-3. the tree gets committed, and
-4. the `HEAD` pointer gets advanced.
+1. The results are updated both in the index file and in your
+ working tree;
+2. Index file is written out as a tree;
+3. The tree gets committed; and
+4. The `HEAD` pointer gets advanced.
Because of 2., we require that the original state of the index
file to match exactly the current `HEAD` commit; otherwise we