[editor] Add flag to changeset when features are merged

[map-per]

Re-added flagging, like in #7853, but for "info:features_merged=yes" instead of "feature_cloes_by".
The "info:features_merged=yes" tag is added to changesets when the feature merging code is executed. This makes evaluating / making statistics for #2298 possible (see: #7853 (comment)).

✔️ Tested in https://www.openstreetmap.org/changeset/151378253

[biodranik]

What's the value in the stats? I've checked the code, it takes any nearby feature in 1 meter radius to match. There is no any other "matching" logic implemented at all, even a simple tags comparison is missing )

It should be easy to implement it though.

[map-per]

I think the flagging makes sense. A merging algorithm that deciding whether to add or merge features is not straightforward / trivial (e.g. if users add the same shop, but with slightly different name or when there are two distinct shops on top of each other on different floors of a shopping centre). So even even when we finally have a good merging algorithm it will make mistakes, so the "info:objects_merged=yes" changesets are still especially interesting for human reviewers. (Without the flag I don't know any way of filtering for merge changeset with standard OSM reviewing software). And for developing the algorithm it is also helpful to not miss out edge cases or try out the algorithm on paper with real world example data. At the moment we don't know how often #2298 occurs and we don't know what common cases there are that a merging algorithm should handle correctly.

[biodranik]

Agree, very good arguments for debugging and improving the algo. Thanks for the PR!