push: fix git push when servers has unknown references
push: fix git push when servers has unknown references

Previously we were trying to do some computation from the unknown objects
advirtised by the server (although they were in the exclude set).
The patch ensure that we just skip these objects instead of failing to produce
a valid path.

The use-case where this could happen is:
1. call fetch_one
2. do some update
3. call update_and_set

In the later case, having only one branch locally would cause some issues
without this patch.

mirage/ocaml-git,mirage/ocaml-git,mirage/ocaml-git