Extend filter syntax with optional specification of file status: add, modified, deleted (#22)

* Add support for specification of change type (add,modified,delete)

* Use NULL as separator in git-diff command output

* Improve PR test workflow

* Fix the workflow file

src/file.ts

@@ -0,0 +1,13 @@
+export interface File {
+  filename: string
+  status: ChangeStatus
+}
+
+export enum ChangeStatus {
+  Added = 'added',
+  Copied = 'copied',
+  Deleted = 'deleted',
+  Modified = 'modified',
+  Renamed = 'renamed',
+  Unmerged = 'unmerged'
+}