Exclude the default branch from being flushed

[BlueRaja]

In repos that are less active, we're seeing the archive for /master being flushed, which is undesirable. Our only option is to increase the timeout for those repos, which means we can't have a corporate-wide standard timeout.

It'd be nice to be able to prevent the default branch's archive from being flushed (or just have the plugin not do it by default)

[tboerger]

You can also exclude the flush step for the master branch