Only ask GitHub for Pull Request commits once
Curry removes former commit authors from PRs

Until now, Curry would only ever add commit authors to its internal
record of each Pull Request. This results in incorrect records when
commit authors disappear from a Pull Request (for example, when commits
from several authors are sqaushed into one). Curry now compares the
post-import set of commit authors to the pre-import set and removes
commit authors who are in the latter but not in the former.
Merge pull request #700 from opscode/curry-removes-commit-authors-if-necessary

Curry removes commit authors if necessary