As I understand you don't want branches/ and tags/ in Git. In this case you can use the following configuration:
trunk = trunk:refs/heads/master
branches = folder1:refs/heads/folder1
branches = folder2:refs/heads/folder2
You can find more information about trunk/branches/tags/shelves options in branches mapping article. Optionally you can add shelves=shelves/*:refs/shelves/*
option if you need bi-directional translation and want to translate every single Git commit to a separate revision in case of self-merges in branches.
Update: the question was about translating trunk and a couple of branches into the same Git branch. I can only propose the following approach:
trunk = :refs/heads/master
includePath = /trunk
includePath = /folder1
includePath = /folder2
It will translate the project root into refs/heads/master but skip everything except these 3 directories. Note that not skipped directories will still be downloaded from SVN but discarded locally later.
This this is the closest solution to your original question, I don't know anything better.