Collection Repeat: Use root scope when checking for in-progress digest

I was having an issue using collection-repeat when a digest was in-progress. The in-progress digest was reflected on the root scope by checking `$$phase`, but not when checking `$$phase` on the child scope that collection-repeat was using. This was causing the ol' "digest already in progress" error.

This pull request is to update collection-repeat to use the root scope when checking for an in-progress digest.

*Note: I tried to recreate this bug in smaller codepen example that I could share but was unfortunately unable to do so. I am actually unsure of the exact conditions that cause this issue.* :-/