"Merge with import data" should update existing feature, rather than create new one

[mikelmaron]

when merging tags from an existing feature, a new feature is created, and the old one also remains. would be better, if just a single merge, to update the existing feature.

[mikelmaron]

https://github.com/mapkibera/osmly/commit/911065ae1820c174e9829a7d8c474518b40764dc

"osmly.settings.merge_preserve": retain osm_ tags, and use to modify existing feature, rather than delete and create new

only works for points. probably fragile