Add --skip-sync to pull-requests-prep

This allow much quicker re-runs of pull-requests-prep if sync has
already been run recently.
Use consistent indentation style
Fix typo in error message
dev: rename cut_feature() to new_feature() for consistency
dev: add some input validation to {new,erase}-feature
Merge pull request #1710 from aspiers/master

Merged by devtool for VictorLowther