Added stashing to pre-commit file

pre-commit.sh

 #!/usr/bin/env bash
 
+git stash --keep-index -q
+
 source ~/.virtualenvs/oscar/bin/activate
 ./run_tests.py
-[ $? -ne 0 ] && echo "Tests failed" && exit 1
+TEST_RESULT=$?
+
+git stash pop -q
+
+[ $TEST_RESULT -ne 0 ] && echo "Tests failed" && exit 1
 
 exit 0