Fix .gitignore. (#102)
Fix .gitignore. (#102)

A leading slash means that the pattern is relative to the directory
level of the .gitignore file.

A trailing slash matches directories -- but bazel-* are symlinks.
world-federation-of-advertisers/panel-exchange-client,world-federation-of-advertisers/panel-exchange-client,world-federation-of-advertisers/panel-exchange-client