Overriding sync-exclude.lst in subfolder

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### Feature description
We have a repository where ``.cmo`` files are not synced because they are generated by an OCaml compiler and tied to the compiler version, which may differ from computer to computer.

However, for a specific project where we generate ``.cmo`` files for fixed versions of the OCaml version (for distribution to students), I would like ``.cmo`` files in that specific subfolder and its subfolders to be synced.

I put an empty ``.sync-exclude.lst`` in that subfolder in the hope that it would take precedence over the one a the repository root, but that does not seem to be the case. I also cannot find in the documentation a way to expressly un-exclude some patterns (which would work because the files to sync are all called ``support.cmo``).

I propose either :

- ``.sync-exclude.lst`` in subfolder taking precedence over those in folders above (even when empty), or
- a syntactic device for an exclusion pattern in a ``.sync-exclude.lst`` to not cover some cases (ie. exclude ``.cmo`` files that are not ``support.cmo``, or exclude ``.cmo`` files that are not ``some/path/*.cmo``).