Repository::commit should use an Option for some of its arguments

[akhilkpdasan]

The libgit2 API says that author and message can be left NULL in git_rebase_commit function to keep the original author and message Doc. However, in git2-rs the commit method on Rebase struct takes values instead of Options for author and message.
I ended up with something like this

while let Some(Ok(op)) = rebase.next() {
    let commit = repo.find_commit(op.id())?;
    let msg = commit.message().unwrap_or("");
    let sig = commit.author();
    rebase.commit(&sig, &sig, msg)?;
}

I couldn't get a for loop to work

for operation in rebase { //value moved here
    let commit = repo.find_commit(operation?.id())?;
    let sig = commit.author();
    let msg = commit.message().unwrap_or("");
    rebase.commit(&sig, &sig, msg)?; //value borrowed here after move
}

[alexcrichton]

Oh hm this may be a mistake in the API! It may be the case that the API should change to taking Option