Fixing corner case where lint doesn't run outside of a PR.

[dhermes]

No description provided.

[coveralls]

Coverage remained the same when pulling 691b9f4 on dhermes:make-lint-run-always into 8e12e10 on GoogleCloudPlatform:master.

[dhermes]

@silvolu@tseaver This should be a quick review.

I realized that git diff origin/master is pretty useless when we're on origin/master so the speedup is only applied during a PR.

To get a sense for what Travis clones / checks out in a PR build, check out the output from a dummy build I made:

$ echo ${TRAVIS_PULL_REQUEST}
2
$
$ git branch -vv
* (detached from FETCH_HEAD) 6c26349 Merge a70ea6b3c3c7df4661436b0dea9b31d651cb9dba into d4c46af11107089cfd8ee8cde1077243ddd3ed7e
master d4c46af [origin/master] Moar spurious blank lines.

[tseaver]

LGTM.