Force load data if the current HEAD has a schema change.

This patch checks the test-warehouse's stored githash (if it exists) to determine if the
current patch has changed the schema if a table. If a change is detected, we force load
all the data.

Change-Id: I314f9f3364d3e6b2d66de38a9e6d9f57c4e279a7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3049
Reviewed-by: Ishaan Joshi <ishaan@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3163