Add --files to log_since.py to prints all the files modified between two revs.
Add --files to log_since.py to prints all the files modified between two revs.

This is useful to quickly glance which files were touched and if a project is
affected by the upgrade.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/150930043

pombreda/swarming,pombreda/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming,luci/luci-py,madecoste/swarming,luci/luci-py,madecoste/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming