Change default diff algorithm to histogram and add tests

The referenced bug showed that JGit produced different merge results
compared to C Git. Unit test was added to reproduce the issue. The
problem can be solved by switching to histogram diff algorithm.

Bug: 331078
Change-Id: I54f30afb3a9fef1dbca365ca5f98f4cc846092e3
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Philipp Thun <philipp.thun@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/merge/MergeAlgorithmTest.java

@@ -165,6 +165,25 @@ public class MergeAlgorithmTest extends TestCase {
 		assertEquals(t("aZcYe"), merge("abcde", "aZcde", "abcYe"));
 	}
 
+	/**
+	 * Test merging two contents which do one similar modification and one
+	 * insertion is only done by one side. Between modification and insertion is
+	 * a block which is common between the two contents and the common base
+	 *
+	 * @throws IOException
+	 */
+	public void testTwoSimilarModsAndOneInsert() throws IOException {
+		assertEquals(t("IAAJ"), merge("iA", "IA", "IAAJ"));
+		assertEquals(t("aBcDde"), merge("abcde", "aBcde", "aBcDde"));
+		assertEquals(t("IAJ"), merge("iA", "IA", "IAJ"));
+		assertEquals(t("IAAAJ"), merge("iA", "IA", "IAAAJ"));
+		assertEquals(t("IAAAJCAB"), merge("iACAB", "IACAB", "IAAAJCAB"));
+		assertEquals(t("HIAAAJCAB"), merge("HiACAB", "HIACAB", "HIAAAJCAB"));
+		assertEquals(t("AGADEFHIAAAJCAB"),
+				merge("AGADEFHiACAB", "AGADEFHIACAB", "AGADEFHIAAAJCAB"));
+
+	}
+
 	private String merge(String commonBase, String ours, String theirs) throws IOException {
 		MergeResult r = new MergeAlgorithm().merge(RawTextComparator.DEFAULT,
 				T(commonBase), T(ours), T(theirs));

org.eclipse.jgit/src/org/eclipse/jgit/merge/MergeAlgorithm.java

@@ -50,24 +50,24 @@ import java.util.List;
 import org.eclipse.jgit.diff.DiffAlgorithm;
 import org.eclipse.jgit.diff.Edit;
 import org.eclipse.jgit.diff.EditList;
-import org.eclipse.jgit.diff.MyersDiff;
+import org.eclipse.jgit.diff.HistogramDiff;
 import org.eclipse.jgit.diff.Sequence;
 import org.eclipse.jgit.diff.SequenceComparator;
 import org.eclipse.jgit.merge.MergeChunk.ConflictState;
 
 /**
  * Provides the merge algorithm which does a three-way merge on content provided
- * as RawText. By default {@link MyersDiff} is used as diff algorithm.
+ * as RawText. By default {@link HistogramDiff} is used as diff algorithm.
  */
 public final class MergeAlgorithm {
 	private final DiffAlgorithm diffAlg;
 
 	/**
-	 * Creates a new MergeAlgorithm which uses {@link MyersDiff} as diff
+	 * Creates a new MergeAlgorithm which uses {@link HistogramDiff} as diff
 	 * algorithm
 	 */
 	public MergeAlgorithm() {
-		this(MyersDiff.INSTANCE);
+		this(new HistogramDiff());
 	}
 
 	/**