Try to set the Project's repo if it does not have one when a new RepositoryProvider is registered.

I tested this using my test `HgRepositoryProvider`. Now when I run the following from the
command line:

    atom <path-to-directory-with-hg-repository>

And then run the following in the console:

    atom.project.getRepositories()

I get an array with an `HgRepository` in it. Previously, I got an empty array because the
`Project`'s paths were set before my `HgRepositoryProvider` was registered.

src/project.coffee

@@ -53,7 +53,15 @@ class Project extends Model
     atom.packages.serviceHub.consume(
       'atom.repository-provider',
       '^0.1.0',
-      (provider) => @repositoryProviders.push(provider))
+      (provider) =>
+        @repositoryProviders.push(provider)
+
+        # If a path in getPaths() does not have a corresponding Repository, try
+        # to assign one by running through setPaths() again now that
+        # @repositoryProviders has been updated.
+        if null in @repositories
+          @setPaths(@getPaths())
+      )
 
     @subscribeToBuffer(buffer) for buffer in @buffers