Fix the formatting checks in Travis-CI (#66)
Fix the formatting checks in Travis-CI (#66)

git clang-format was checking changes outside of the change when a merge
was in the middle of the pull request. It turns out that Travis-CI is
checking out a merge commit for each pull request, so we need to use the
--diff flag of git clang-format.

Also only run the checks for pull requests.
google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt,google/conscrypt