Clone rather than edit

[UnimelbLachlan]

Please introduce a mode that performs a fresh clone, and then filters the fresh clone, rather than editing in-place.

The advantages are:

1. This is more reliable than using heuristics to check that this is a fresh clone
2. Users doesn't have to perform a compulsory step first (possibly incorrectly, if --no-local was omitted, like I just did).
3. It allows for optimisation: rather than doing a complete clone and then deleting, it would be possible in future simply to create the requested files in the clone process and omit all others. This doesn't have to be implemented yet.
4. It gets people out of the mindset of "it is OK to modify a repo" (which goes against the spirit of version control) into the mindset of "Let's create a new repo based on an old one" which is totally legitimate.