1. Download all old sources from snapshot.debian.org: debsnap packagename
2. Create git repository and cd into it: mkdir $packagename && cd $packagename && git init
3. Tell git to merge changes from new versions automatically: git config branch.master.mergeoptions '-s recursive -X theirs'
4. Import the sources as listed in the .dsc files: git-import-dscs ../*.dsc
5. Unset the merge options: git config –unset branch.master.mergeoptions

It might be useful to patch git-import-dscs to set branch.master.mergeoptions automatically, but there might be reasons where you want to avoid that (for example when uptsream shipped a debian folder and the maintainer did not remove it).