[SPARK-2912] [Spark QA] [WIP] Include commit hash in Spark QA messages

You can find the [discussion that motivated this PR here](http://mail-archives.apache.org/mod_mbox/spark-dev/201408.mbox/%3CCABPQxssy0ri2QAz=cc9Tx+EXYWARm7pNcVm8apqCwc-esLbO4Q@mail.gmail.com%3E). Note that this is a work in progress and will be replaced by a cleaner PR.

As described in [SPARK-2912](https://issues.apache.org/jira/browse/SPARK-2912), the goal of this PR (and related ones to come) is to include useful detail in Spark QA's messages that are intended to make a committer's job easier to do.

Since this work depends on Jenkins, I cannot test this locally. Hence, I will be iterating via this PR.

[Note: This is a duplicate of a [previous PR](https://github.com/apache/spark/pull/1811), without the extraneous commits.]
Actually, on that note, do we even want to run tests at all if the patch doesn't merge cleanly? What's the point? Maybe it would be better if Jenkins complained right away and said he wasn't gonna test the patch until it merged cleanly. What do you say @pwendell?