Simplify getting dirty files in repository

[clyvari]

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

Background: When getting a RepositoryStatus, we can use the IsDirty 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 the RepositoryStatus ctor to initialize the IsDirty prop.

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

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

2. Least importantly, but I think it's nice, we add a Dirty collection inside RepositoryStatus, that is initialized with all the IsDirty StatusEntries

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

if(repositoryStatus.IsDirty)
{
    var dirtyFiles = 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)
{
    var dirtyFiles = repositoryStatus.Dirty;
}

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