Optimize compare of local to remote commits

[jacekv]

In some places, like fetch, we are downloading all remote commits and need to unpack them. Unpacking means, changing from IPFS structure to git structure.
In order to not override already existing git objects, we are reading all local commits and save the hashes into a set. The same happens with the remote commits.
Than we substract those sets from each other in order to get the difference between remote and local commits.

This can be improved! Instead of reading all local commits, we should just read the current head of a branch. Using this head hash, we can cut of the remote commits, which we don't need to load!

This will save time and disk access!