LocalFileHistoryFinder.GetManifestFilenames() should return relative paths

[doughertym]

When the LocalFileHistoryFinder is used, the GetManifestFilenames() currently returns just the filename. However, this can cause issues if there are manifest files located further down the tree than the top-level. For example corgibytes/freshli-fixture-csharp-test:directory/TestProjectInDirectory.csproj will be returned as TestProjectInDirectory.csproj.