ENH: enable lazy copy in merge() for CoW

[jorisvandenbossche]

xref #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)

[phofl]

lgtm.

As a follow up:

tests for left/right merges where only one side isn't copied

[phofl]

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]

[phofl]

thx @jorisvandenbossche