Clarify the copy-existing-baselines-internal command.

Background: The "webkit-patch copy-existing-baselines-internal" command is one of the parts of the layout test rebaseline process which I found a little hard to understand at first. I edited the test earlier, but I think that the code and test could still be made clearer.
Review-Url: https://codereview.chromium.org/2721663003
Cr-Commit-Position: refs/heads/master@{#453760}
3 files changed