Googling around a bit, I found the solution which I'm going to share here with you. It involves a couple of steps.
- Remove the .git directories from the external bundles
git filter-branch --index-filter 'git rm -r -f --cached --ignore-unmatch vendor/bundles/Liip/MonitorBundle' -fon each of the external bundle directories where you removed the .git directory. This command needs to be run from the root of your git project!
- Git svn dcommit your changes again
Some links I used while researching this are: