Garmin doesnt offer a changelog for map updates. It's impossible to list in detail all the changes for new roads and changes to existing roads,for the whole Europe or other regions.
Garmin offers a Map viewer in it's map product page:
Code:
Please Login or Register to see the links
Look at Coverage Details tab.
[Only registered and activated users can see links. ]

It's an interactive map, you can zoom to any area and check for changes.
You can also compare the new map Coverage Details with the ones of previous one