2.5.1 Upgrading a Buildmaster to Buildbot-0.7.6

The 0.7.6 release introduced the public_html/ directory, which contains index.html and other files served by the WebStatus and Waterfall status displays. The upgrade-master command will create these files if they do not already exist. It will not modify existing copies, but it will write a new copy in e.g. index.html.new if the new version differs from the version that already exists.