Add validation for parsed files #12

If a patch is malformed or a File is created directly, various fields may disagree. Add a validate function that checks for these types of issues so clients (e.g. appliers) can rely on the content of the fields.

Some of the issues to check:

• IsRename is true/false but OldName and NewName are equal/not equal
• IsDelete or IsNew is true but there is more than one fragment
• IsDelete is true but the single fragment has context or addition lines or NewPosition and NewLines are not 0
• IsNew is true but the single fragment has context or deletion lines or OldPosition and OldLines are not 0
• IsBinary is true but TextFragments is not empty