[io] Update the TTree behavior to use the first TTree compression when the flag "FirstSrcCompression" is present

[silverweed]

Explain what you would like to see improved and how.

We should update the TTree merging code to honor the newly introduced "FirstSrcCompression" TFileMerger flag.
See here

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Installation method

Operating system

Additional context

No response

[ferdymercury]

Do you mean the code TTree::Merge ? Isn't that already cloning the first TTree by default (and then copying the entries to it?)

EDIT: Ok, so you mean to make it more consistent with RNtuple's behavior:
#17299 (comment)

[silverweed]

If that's the case maybe there is nothing else to do @pcanal ?