Use w1 for hashCode of AbbreviatedObjectId

Very short abbreviations that are under 8 hex digits do not
have values in w2. Use w1 as the Java hashCode() instead, so
that the prefix of the abbreviation is always included in the
hashing function used by any java.util.Collection type.

Change-Id: Idaf69f86b62630ba4a022d31b4c293c6d138f557

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

@@ -86,7 +86,6 @@ public class AbbreviatedObjectIdTest {
 		final ObjectId f = i.toObjectId();
 		assertNotNull(f);
 		assertEquals(ObjectId.fromString(s), f);
-		assertEquals(f.hashCode(), i.hashCode());
 	}
 
 	@Test
@@ -101,7 +100,6 @@ public class AbbreviatedObjectIdTest {
 		final ObjectId f = i.toObjectId();
 		assertNotNull(f);
 		assertEquals(ObjectId.fromString(s), f);
-		assertEquals(f.hashCode(), i.hashCode());
 	}
 
 	@Test
@@ -215,7 +213,7 @@ public class AbbreviatedObjectIdTest {
 	}
 
 	@Test
-	public void testEquals_Short() {
+	public void testEquals_Short8() {
 		final String s = "7b6e8067";
 		final AbbreviatedObjectId a = AbbreviatedObjectId.fromString(s);
 		final AbbreviatedObjectId b = AbbreviatedObjectId.fromString(s);
@@ -225,6 +223,18 @@ public class AbbreviatedObjectIdTest {
 		assertEquals(a, b);
 	}
 
+	@Test
+	public void testEquals_Short4() {
+		final String s = "7b6e";
+		final AbbreviatedObjectId a = AbbreviatedObjectId.fromString(s);
+		final AbbreviatedObjectId b = AbbreviatedObjectId.fromString(s);
+		assertNotSame(a, b);
+		assertTrue(a.hashCode() != 0);
+		assertTrue(a.hashCode() == b.hashCode());
+		assertEquals(b, a);
+		assertEquals(a, b);
+	}
+
 	@Test
 	public void testEquals_Full() {
 		final String s = "7b6e8067ec96acef9a4184b43210d583b6d2f99a";

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

@@ -336,7 +336,7 @@ public final class AbbreviatedObjectId implements Serializable {
 
 	@Override
 	public int hashCode() {
-		return w2;
+		return w1;
 	}
 
 	@Override