cargo: use bundled libgit2

Cargo uses git-rs which is made to be built against the bundled libgit2
version that hasn't been part of a stable release yet. Using our libgit2
instead of the master version fails during runtime as they are not
compatible anymore.

After the next libgit2 update we can try again but it is likely that
there will also be yet another cargo release at that point in time…