Fix resolving expression with ~ and ^ than extends beyond history

resolve("foo~X") where X is greater than the distance from foo to the
root should return null, but 2a2362fb introduced a bug causing it to
either return resolve("foo") or NPE. Add a test for the correct
behavior.

Also add an analogous test for foo^X where X is greater than the
number of parents (which was not broken by that commit).

Change-Id: Ic580081ece57c8c2df29b652897b425ecb34e11f

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/RepositoryResolveTest.java

@@ -99,6 +99,15 @@ public class RepositoryResolveTest extends SampleDataRepositoryTestCase {
 		assertEquals("d0114ab8ac326bab30e3a657a0397578c5a1af88",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0^03").name());
 	}
 
+	@Test
+	public void testObjectId_objectid_invalid_explicit_parent() throws IOException {
+		assertEquals("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1",db.resolve("6462e7d8024396b14d7651e2ec11e2bbf07a05c4^1").name());
+		assertNull(db.resolve("6462e7d8024396b14d7651e2ec11e2bbf07a05c4^2"));
+		assertEquals("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1",db.resolve("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1^0").name());
+		assertNull(db.resolve("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1^1"));
+		assertNull(db.resolve("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1^2"));
+	}
+
 	@Test
 	public void testRef_refname() throws IOException {
 		assertEquals("49322bb17d3acc9146f98c97d078513228bbf3c0",db.resolve("master^0").name());
@@ -120,6 +129,20 @@ public class RepositoryResolveTest extends SampleDataRepositoryTestCase {
 		assertEquals("1203b03dc816ccbb67773f28b3c19318654b0bc8",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0~~~0").name());
 	}
 
+	@Test
+	public void testDistance_past_root() throws IOException {
+		assertEquals("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1",db.resolve("6462e7d8024396b14d7651e2ec11e2bbf07a05c4~1").name());
+		assertNull(db.resolve("6462e7d8024396b14d7651e2ec11e2bbf07a05c4~~"));
+		assertNull(db.resolve("6462e7d8024396b14d7651e2ec11e2bbf07a05c4~2"));
+		assertNull(db.resolve("6462e7d8024396b14d7651e2ec11e2bbf07a05c4~99"));
+		assertNull(db.resolve("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1~~"));
+		assertNull(db.resolve("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1~2"));
+		assertNull(db.resolve("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1~99"));
+		assertEquals("42e4e7c5e507e113ebbb7801b16b52cf867b7ce1",db.resolve("master~6").name());
+		assertNull(db.resolve("master~7"));
+		assertNull(db.resolve("master~6~"));
+	}
+
 	@Test
 	public void testTree() throws IOException {
 		assertEquals("6020a3b8d5d636e549ccbd0c53e2764684bb3125",db.resolve("49322bb17d3acc9146f98c97d078513228bbf3c0^{tree}").name());

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -532,6 +532,7 @@ public abstract class Repository {
 					if (name == null)
 						name = new String(revChars, done, i);
 					rev = parseSimple(rw, name);
+					name = null;
 					if (rev == null)
 						return null;
 				}
@@ -588,6 +589,7 @@ public abstract class Repository {
 							// detached
 							name = Constants.HEAD;
 						Ref ref = getRef(name);
+						name = null;
 						if (ref == null)
 							return null;
 						if (ref.isSymbolic())
@@ -635,6 +637,7 @@ public abstract class Repository {
 						if (name.equals(""))
 							name = Constants.HEAD;
 						Ref ref = getRef(name);
+						name = null;
 						if (ref == null)
 							return null;
 						// @{n} means current branch, not HEAD@{1} unless
@@ -642,7 +645,6 @@ public abstract class Repository {
 						if (ref.isSymbolic())
 							ref = ref.getLeaf();
 						rev = resolveReflog(rw, ref, time);
-						name = null;
 					}
 					i = m;
 				} else
@@ -656,6 +658,7 @@ public abstract class Repository {
 					if (name.equals(""))
 						name = Constants.HEAD;
 					rev = parseSimple(rw, name);
+					name = null;
 				}
 				if (rev == null)
 					return null;