IMPALA-5482: fix git checkout when workloads are modified
IMPALA-5482: fix git checkout when workloads are modified

When git checkout would overwrite changes, it fails and alerts the
user to do something with the changes. This patch removes any changes
to files induced by the workload copy-and-paste.

Testing: using a patch provided by Lars Volker that touched
testdata/workloads/ (https://gerrit.cloudera.org/#/c/7073/), I was
able to reproduce the problem he saw and see that this patch fixed it.

Change-Id: I9a0d004c353eb4b547aeaf3c56289594326653d7
Reviewed-on: http://gerrit.cloudera.org:8080/7145
Reviewed-by: Lars Volker <4d73c29a4561048ce85fa0051bb7a6d201781159@cloudera.com>
Tested-by: Impala Public Jenkins

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala