[MERGE] 40% faster partial commits

[Ian Clatworthy]

I'm 85% of the way through reviewing this and all looks good so far.
commit.py to be completed and I want to do that in the morning (when my
mind is fresher).

Comments so far below.

bb:comment

> +    search_paths = set()
> +    paths = set(paths)
> +    for path in paths:
> +        other_paths = paths.difference(set([path]))
> +        if not is_inside_any(other_paths, path):
> +            # this is a top level path, we must check it.
> +            search_paths.add(path)
> +    return search_paths

difference takes an iterable so no need for set([path]). Just [path]
ought to do.

Ian C.