tweak keywords checking logic

Just backend improvements:
 - use more standardized func name
 - use _filter_files rather than ad-hoc implementation

BUG=chromium:340036
TEST=`./pre-upload_unittest.py` passes

Change-Id: Ia76364fbf1a7dc1d42e19554126d1ae61193feba
Reviewed-on: https://chromium-review.googlesource.com/184624
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed