git reset before checkout in scripts

This is a temporary fix for allowing
`git checkout` to work in our perf presubmit
scripts which fail because of earlier `chmod +x`
calls which make those shell scripts modified and
that fails the next `git checkout` calls.

This is a temporary fix, which will be removed in next change
which will remove chmod calls from all script files.

perfmetrics/scripts/presubmit_test/pr_perf_test/build.sh

@@ -88,6 +88,7 @@ if [[ "$perfTestStr" == *"$EXECUTE_PERF_TEST_LABEL"* ]];
 then
  # Executing perf tests for master branch
  install_requirements
+ git reset --hard
  git checkout master
  # Store results
  touch result.txt
@@ -97,6 +98,7 @@ then
 
  # Executing perf tests for PR branch
  echo checkout PR branch
+ git reset --hard
  git checkout pr/$KOKORO_GITHUB_PULL_REQUEST_NUMBER
  echo Mounting gcs bucket from pr branch and execute tests
  execute_perf_test
@@ -110,6 +112,7 @@ fi
 if [[ "$integrationTestsStr" == *"$EXECUTE_INTEGRATION_TEST_LABEL"* ]];
 then
   echo checkout PR branch
+  git reset --hard
   git checkout pr/$KOKORO_GITHUB_PULL_REQUEST_NUMBER
 
   echo "Running e2e tests...."