ENH: enable lazy copy in merge() for CoW
xref https://github.com/pandas-dev/pandas/issues/49473

The current `copy=False` only works when merging on the index; when merging on keys, we currently get a [0, 1, 2, ..] join index for both left and right, and so that still results in a copy (but that can be avoided using the `is_range_indexer` utility)
Ah mypy is complaining:

```
pandas/core/reshape/merge.py:155: error: Argument "copy" to "get_result" of "_MergeOperation" has incompatible type "Optional[bool]"; expected "bool"  [arg-type]
```  thx @jorisvandenbossche 