added explicit dirty flag to output

I found myself needing to know both the base commit hash, and whether or not the repository was dirty at the time of the build, so I ended up parsing out the dirty flag from the commit id.

This change makes this case easier to handle; you can even set the dirtyFlag string to the empty string, and then the only indication of dirtiness will be this new property.

This change doesn't alter any existing behavior, so if you don't care to look at this new property, it shouldn't interfere with your build.