Merging gas station and roof removes the roof tag

[bhousel]

As reported on OSM-US Slack:
https://osmus.slack.com/archives/C029HV951/p1498113350658018

Using iD 2.2.2, I'm merging a node with tags amenity=fuel and name=Costco Gas, with a closed way with building=roof. I expected the resulting closed way to have all three tags, but it dropped building=roof. Is this expected? I think iD's behavior earlier was to merge all three tags.

This is related to #4113, in that it's the same kind of merge. The building=roof tag shouldn't be discarded after merging.

[bhousel]

Fix for this was a little weird, but the point-merge operation handles tags a little better now.

After #3851 we are now rematching the preset after a merge.
With this fix we now:

• don't remove the old preset's tags (which was over-aggressively removing the building=roof),
• do remove any existing area=yes when adding the tags for the new preset,
  (just before the code that decides whether an area=yes tag is really necessary).