gh-pages target is slow

[twpayne]

Currently, building the gh-pages target to update the GitHub pages is really slow because it clones the full ol3 repository. It can be made faster by first fetching from the existing local repo and then fetching from the gh-pages branch.

[twpayne]

Just bumping this, because updating gh-pages over a slow connection sucks.

[ahocevar]

@twpayne If you want to get online examples for master, I'd suggest cloning https://github.com/openlayers/openlayers.github.io and running grunt. This is much faster and you get the examples at http://ol3js.org/en/master/examples. The reason why it is faster is because it only runs the host-examples target and does less git shuffling than the gh-pages target in build.py.

[twpayne]

Thanks @ahocevar ! Will use this in future.

[fredj]

See #839