Do not attempt to resolve describe-labels with less than four digits

Change-Id: I21dcd3cca3b41102fd898238d8d640dea25e0caf
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

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

@@ -191,6 +191,10 @@ public class RepositoryResolveTest extends SampleDataRepositoryTestCase {
 
 		assertEquals(id, db.resolve("refs/heads/foo-g032c"));
 		assertEquals(id, db.resolve("foo-g032c"));
+		assertNull(db.resolve("foo-g032"));
+		assertNull(db.resolve("foo-g03"));
+		assertNull(db.resolve("foo-g0"));
+		assertNull(db.resolve("foo-g"));
 
 		ru = db.updateRef("refs/heads/foo-g032c-dev");
 		ru.setNewObjectId(id);

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

@@ -599,7 +599,7 @@ public abstract class Repository {
 			return resolveAbbreviation(revstr);
 
 		int dashg = revstr.indexOf("-g");
-		if ((dashg + 4) < revstr.length() && 0 <= dashg
+		if ((dashg + 5) < revstr.length() && 0 <= dashg
 				&& isHex(revstr.charAt(dashg + 2))
 				&& isHex(revstr.charAt(dashg + 3))
 				&& isAllHex(revstr, dashg + 4)) {