NameRevCommand: Use ~ notation for first parents of merges

Prefer ~(N+1) to ^1~N. Although both are correct, the former is
cleaner and matches "git name-rev".

Change-Id: I772001a219e5eb346f5552c92e6d98c70b2cfa98

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/NameRevCommandTest.java

@@ -144,7 +144,21 @@ public class NameRevCommandTest extends RepositoryTestCase {
 		RevCommit c2 = tr.commit().parent(c0).create();
 		RevCommit c3 = tr.commit().parent(c1).parent(c2).create();
 		tr.update("master", c3);
-		assertOneResult("master^1~1", c0);
+		assertOneResult("master~2", c0);
+	}
+
+	@Test
+	public void onePathMergeSecondParent() throws Exception {
+		// 0--1-----4
+		//  \-2--3-/
+		RevCommit c0 = tr.commit().create();
+		RevCommit c1 = tr.commit().parent(c0).create();
+		RevCommit c2 = tr.commit().parent(c0).create();
+		RevCommit c3 = tr.commit().parent(c2).create();
+		RevCommit c4 = tr.commit().parent(c1).parent(c3).create();
+		tr.update("master", c4);
+		assertOneResult("master^2", c3);
+		assertOneResult("master^2~1", c2);
 	}
 
 	@Test

org.eclipse.jgit/src/org/eclipse/jgit/api/NameRevCommand.java

@@ -147,12 +147,11 @@ public class NameRevCommand extends GitCommand<Map<ObjectId, String>> {
 					break;
 				if (c.getCommitTime() < cutoff)
 					continue;
-				boolean merge = c.getParentCount() > 1;
+				long cost = c.cost + (c.getParentCount() > 1 ? MERGE_COST : 1);
-				long cost = c.cost + (merge ? MERGE_COST : 1);
 				for (int i = 0; i < c.getParentCount(); i++) {
 					NameRevCommit p = (NameRevCommit) walk.parseCommit(c.getParent(i));
 					if (p.tip == null || compare(c.tip, cost, p.tip, p.cost) < 0) {
-						if (merge) {
+						if (i > 0) {
 							p.tip = c.format().append('^').append(i + 1).toString();
 							p.distance = 0;
 						} else {