change github api for commits in pulls

apparently github changed their api again
this commit updates `pullrequestcommits` to return a list of `PRCommit`s
this type was formerly called `CommitInfo`
a new class `CommitInfo` is introduced as the type
for `PRCommit`'s `commit` member

also remove unused fields to reduce our exposure to future api changes

review by @jsuereth, @dotta