Add ability to ignore commits from the gh-pages branch

[ElliotSilver]

Commits on any branch trigger the ig auto-build. I am publishing my website on the gh-pages branch of my repository, and don't want to trigger the auto-build when that content changes. (I expect any build on that branch is going to fail.) It would be great to be able for the auto-build to ignore the gh-pages branch (or to somehow specify which branches to ignore, for example with the presence of a "no-ig-build" file in the root directory of the branch).

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