Speed up external templates

[kasperg]

If using a template from a git url the template appears to be cloned each time the site is generarted. That makes the preview process slower.

It would be nice if we could keep using the the checkout if the url does not change. To ensure we use the latest version we could pull.

[mnapoli]

👍

Pulling doesn't even seem necessary to me, that would probably be as slow (or close) and wouldn't bring much. Restarting the preview seems like an acceptable compromise for ease of use.

Should be fixed when #58 is merged.

[kasperg]

Sounds great.