Clone only with depth 1

[xkjyeah]

This should speed up cloning significantly for gh-pages with long histories

[xkjyeah]

#48

[tschaub]

Thanks for the contribution. It looks like your change would make the existing depth option add a redundant (and potentially conflicting) --depth arg.

I think it makes sense to default to --depth 1, but the existing depth option needs to be considered.

[tschaub]

I've implemented this in an alternative way in #168. This will be included in the next release.