Walk through files in parse_llvm_coverage.py instead of using 'git ls-files'

The script will be executed on a swarming bot where we do not want to isolate .git directories.

BUG=skia:5159
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1888423002

NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1888423002
1 file changed