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
• Run

  git filter-branch --index-filter 'git rm -r -f --cached --ignore-unmatch vendor/bundles/Liip/MonitorBundle' -f

  on 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

Now it should all work again.

Some links I used while researching this are:

• SO: git svn dcommit not work
• A quick trip to the panic room
• Detach many subdirectories into a new, separate Git repository
• How do I enable shell options in git?