Repository::commit should accept parents as &[Commit], not just &[&Commit]

If you want to generate a list of commits to pass to .commit as parents, the type &[&Commit] proves inconvenient, because it requires the caller to maintain ownership of the Commit objects for long enough to pass them. For instance, this could require generating a Vec<Commit>, and then an additional Vec<&Commit>.

Could Repository::commit accept a &[T] where T=AsRef, instead? That would allow both possibilities. It does, however, have the disadvantage that passing &[] will result in an ambiguous type, though. Another possibility would be always accepting &[Commit], but that would break existing code that passes &[&Commit].

I'd welcome other suggestions that would simplify code like this:

    let mut parents : Vec<git2::Commit> = Vec::new();
    parents.extend(tree.iter().filter_map(|e| {
        if e.kind() == Some(git2::ObjectType::Commit) {
            Some(repo.find_commit(e.id()).unwrap_or_else(die))
        } else {
            None
        }
    }));
    let new_commit_oid = repo.commit(Some("SHEAD"), &author, &committer, &msg, &tree, &parents).unwrap_or_else(die);

Start at Repository::commit and inspect its current parent argument and existing callers. Compare the proposed parent representations with compatibility concerns, especially the ambiguous empty-slice case. Done means an agreed API design that supports the intended parent collections without breaking existing callers, with the relevant behavior covered by tests.