Fix ds.merge to prevent altering original object depending on join value

[jsignell]

So far this PR just adds some tests demonstrating how Using certain join values ("exact" and "override") on merge can alter the original objects.

I discovered this in the context of #10062 (comment) but it's not really related to changes in that PR.

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[jsignell]

@dcherian I am taking a look and will report back findings.

[dcherian]

Thanks! we have had this kind of issue before: #10330 Maybe a helper function is in order

[jsignell]

#7736 mentions regression so we will want to make sure that we don't solve this by just copying aggressively.

[jsignell]

Thanks for the pointer to #10330! I just took that logic and moved it outside the if block so that everybody has to go through it. I think the remaining tests are the same that are failing on main.

[jsignell]

See #10599 for fixes to doctests and mypy.

[jsignell]

It would probably be a good idea to run benchmarks on this PR.