Ignore files/directory that are not tracked by git when generating BUILD.gn for Telemetry
Ignore files/directory that are not tracked by git when generating BUILD.gn for Telemetry

Previously, if developers working on catapult/ patches have files or directories in the
top level catapult directory that are untracked by git, generate_telemetry_build.py script
will include those files in BUILD.gn. This CL makes sure that the script
ignores those files instead.

Test: patch this CL, add random local files into catapult/,
run ./generate_telemetry_build.py --> expect that BUILD.gn doesn't change

Bug: chromium:879554
Change-Id: I2c05bacd09360685002273be6e8a8be5ae79c577
Reviewed-on: https://chromium-review.googlesource.com/1213349
Commit-Queue: Ned Nguyen <8e49e083cfcfb68517585b2e78d68c2fd2b70562@google.com>
Reviewed-by: Kenneth Russell <1ebccd90c2bc76de65b6f830f2128a91ed69fdfb@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult