clone, push, pull sometimes fails

[hannesm]

hi, while using this library in a mirageos unikernel which pulls (via tcp, smart git protocol), the initial clone works fine \o/
but if in a separate process i push (using shell and c-git) to the git repository, and pull with ocaml-git this sometimes fails..

i tried to reproduce with an empty repository, but that seems to work fine. I'll hopefully find more time to debug this further and present you with a repository, a local clone, and a failing pull.

[hannesm]

i failed to reproduce this... will close for now, and open a new issue with more detail if i manage to reproduce this behaviour