ipfs-cap2pfs: Drop embedded commits from Git departures

Git has gitlinks (it uses them for submodules [1]).  ls-tree renders
them as:

  160000 commit 907dd9bed8b42be9df419150ed149a3670bbedb6  sci-libs/pycomedi

[1]: http://git-scm.com/docs/git-submodule#_description