Improve fragment validation, track context lines

Track leading and trailing context lines because it's easy and Git
includes this information in the fragment type. Add validation for when
the fragment does not agree with the header or with the new/deleted
state of the file.

gitdiff/gitdiff.go

@@ -39,6 +39,9 @@ type Fragment struct {
 	LinesAdded   int64
 	LinesDeleted int64
 
+	LeadingContext  int64
+	TrailingContext int64
+
 	Lines []FragmentLine
 }