Ensure the uniqueness of DiffNode.getID

Given the diff (without modifications)
```diff
 #ifdef A
 Code
 #endif
```

The ID of the root node and the `#ifdef A` node computed the same ID.
For ensuring the uniqueness of the root node ID its line number is set
to `DiffLineNumber.Invalid` (for consistency both start and end are
changed).

Author: ibbem

src/main/java/org/variantsync/diffdetective/diff/difftree/DiffNode.java

@@ -111,7 +111,7 @@ public static DiffNode createRoot() {
         return new DiffNode(
                 DiffType.NON,
                 NodeType.IF,
-                new DiffLineNumber(1, 1, 1),
+                DiffLineNumber.Invalid(),
                 DiffLineNumber.Invalid(),
                 FixTrueFalse.True,
                 new ArrayList<>()
@@ -982,6 +982,7 @@ public boolean isAnnotation() {
      * {@code 2^26}, this id will no longer be unique.
      */
     public int getID() {
+        // Add one to ensure invalid (negative) line numbers don't cause issues.
         int lineNumber = 1 + from.inDiff;
         Assert.assertTrue((lineNumber << 2*ID_OFFSET) >> 2*ID_OFFSET == lineNumber);
 

src/main/java/org/variantsync/diffdetective/diff/difftree/parse/DiffTreeParser.java

@@ -195,8 +195,6 @@ public static DiffResult<DiffTree> createDiffTree(
             lastArtifact = endCodeBlock(lastArtifact, lineNo);
         }
 
-        endCodeBlock(root, lineNo);
-
         // Invalidate line numbers according to edits.
         // E.g. if a node was added, it had no line number before the edit.
         for (final DiffNode node : nodes) {

src/test/java/TestLineNumbers.java

@@ -22,7 +22,7 @@ public void initTestCases() {
         // Testcases rely on stability of IDs
 
         final var elifchain_map = new HashMap<Integer, Pair<DiffLineNumber, DiffLineNumber>>();
-        elifchain_map.put(144, new Pair<>(new DiffLineNumber(1, 1, 1), new DiffLineNumber(11, 9, 10)));
+        elifchain_map.put(16, new Pair<>(DiffLineNumber.Invalid(), DiffLineNumber.Invalid()));
         elifchain_map.put(147, new Pair<>(new DiffLineNumber(1, 1, 1), new DiffLineNumber(2, 2, 2)));
         elifchain_map.put(208, new Pair<>(new DiffLineNumber(2, 2, 2), new DiffLineNumber(4, 4, 4)));
         elifchain_map.put(275, new Pair<>(new DiffLineNumber(3, 3, 3), new DiffLineNumber(4, 4, 4)));
@@ -35,7 +35,7 @@ public void initTestCases() {
         TestCase elifchain = new TestCase("elifchain.txt", elifchain_map);
 
         final var lineno1_map = new HashMap<Integer, Pair<DiffLineNumber, DiffLineNumber>>();
-        lineno1_map.put(144, new Pair<>(new DiffLineNumber(1, 1, 1), new DiffLineNumber(17, 14, 12)));
+        lineno1_map.put(16, new Pair<>(DiffLineNumber.Invalid(), DiffLineNumber.Invalid()));
         lineno1_map.put(147, new Pair<>(new DiffLineNumber(1, 1, 1), new DiffLineNumber(2, 2, 2)));
         lineno1_map.put(208, new Pair<>(new DiffLineNumber(2, 2, 2), new DiffLineNumber(4, 4, 4)));
         lineno1_map.put(275, new Pair<>(new DiffLineNumber(3, 3, 3), new DiffLineNumber(4, 4, 4)));
@@ -51,7 +51,7 @@ public void initTestCases() {
         TestCase lineno1 = new TestCase("lineno1.txt", lineno1_map);
 
         final var deleteMLM_map = new HashMap<Integer, Pair<DiffLineNumber, DiffLineNumber>>();
-        deleteMLM_map.put(144, new Pair<>(new DiffLineNumber(1, 1, 1), new DiffLineNumber(5, 5, 1)));
+        deleteMLM_map.put(16, new Pair<>(DiffLineNumber.Invalid(), DiffLineNumber.Invalid()));
         deleteMLM_map.put(136, new Pair<>(new DiffLineNumber(1, 1, -1), new DiffLineNumber(4, 4, -1)));
         deleteMLM_map.put(267, new Pair<>(new DiffLineNumber(3, 3, -1), new DiffLineNumber(4, 4, -1)));
         TestCase deleteMLM = new TestCase("deleteMLM.txt", deleteMLM_map);