Only presubmit warn when spammy logging is being added/modified.

Sky has a bunch of printfs that are only used for efficient
local debugging. Don't presubmit warn when a file that contains
one of these is modified unless the patch actually modifies them.

Do this by changing the presubmit to only look at the "+"
lines in the diff (which is what ChangedContents returns).

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/787013005