A simple PR to help getting the dirty files in the repository.

Background: When getting aRepositoryStatus, we can use theIsDirty property to tell if the repository is dirty.
However, we don't have the same convenience to get the list of those dirty files.

Indeed, getting the actual list of dirty files would imply some duplicated logic in the client code, the same code that is present in theRepositoryStatus ctor to initialize theIsDirty prop.

What I propose is two parts (and 2 independent commits):

1. Most importantly, we move theIsDirty detection inside a StatusEntry, by adding a property of the same name. Before, the detection was done inside theRepositoryStatus ctor, analyzing each StatusEntryFileStatus. Now each StatusEntry has the responsibility of telling whether it is dirty.

2. Least importantly, but I think it's nice, we add aDirty collection insideRepositoryStatus, that is initialized with all theIsDirty StatusEntries

Before thos changes, a client code working with dirty files could look like this:

if(repositoryStatus.IsDirty){vardirtyFiles=repositoryStatus.Where(file=>file.State!=FileStatus.Ignored&&file.State!=FileStatus.Unaltered)// Duplicated Logic}

With those two modifications, the client code would look like this:

if(repositoryStatus.IsDirty){vardirtyFiles=repositoryStatus.Dirty;}

I appreciate any feedback, let me know if anything need to be changed.