check_syntax/check_style: more efficient way of finding modified files/modules

Signed-off-by: Dave Simons <dave@inuits.eu>
check_syntax/check_style: make sure all pushed commits are checked, not just the last

Signed-off-by: Dave Simons <dave@inuits.eu>
check_syntax/check_style: initialize $GIT_PREVIOUS_COMMIT if not present

Signed-off-by: Dave Simons <dave@inuits.eu>
Merge pull request #7 from simonsd/master

check all commits since previous job - use GIT_PREVIOUS_COMMIT