diff: Optimize single line edits

If the header and trailer are identical up to a single line on both
sides, return that REPLACE edit as the only result.  No algorithm can
break down a REPLACE with height of 1.

Change-Id: I483c40e8790cc3e8b322ef6dfce2299491fd0ac7

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffAlgorithm.java

@@ -114,6 +114,9 @@ public abstract class DiffAlgorithm {
 			return EditList.singleton(region);
 
 		case REPLACE: {
+			if (region.getLengthA() == 1 && region.getLengthB() == 1)
+				return EditList.singleton(region);
+
 			SubsequenceComparator<S> cs = new SubsequenceComparator<S>(cmp);
 			Subsequence<S> as = Subsequence.a(a, region);
 			Subsequence<S> bs = Subsequence.b(b, region);

org.eclipse.jgit/src/org/eclipse/jgit/diff/HistogramDiff.java

@@ -192,6 +192,9 @@ public class HistogramDiff extends LowLevelDiffAlgorithm {
 				break;
 
 			case REPLACE:
+				if (r.getLengthA() == 1 && r.getLengthB() == 1)
+					edits.add(r);
+				else
 					diffReplace(r);
 				break;