Allow '@' as last character of ref

Previously @ was allowed e.g. in branch names, but not as the last
character. The case that @ is the last character was not handled.

Change-Id: Ic33870b22236f7a5ec7b54007f1b0cefd9354bfb

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

@@ -324,6 +324,8 @@ public class RepositoryResolveTest extends SampleDataRepositoryTestCase {
 		assertFalse(Repository.isValidRefName("x/a\\b"));
 		assertFalse(Repository.isValidRefName("x/a\u0000"));
 
+		db.resolve("x/a@");
+
 		assertUnparseable(".");
 		assertUnparseable("x@{3");
 		assertUnparseable("x[b");

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

@@ -686,6 +686,8 @@ public abstract class Repository implements AutoCloseable {
 			case '@':
 				if (rev != null)
 					throw new RevisionSyntaxException(revstr);
+				if (i + 1 == revChars.length)
+					continue;
 				if (i + 1 < revChars.length && revChars[i + 1] != '{')
 					continue;
 				int m;