Minor improvements to JGitLightweightTagTest

Prepare for JGit 7.2.0 where the `packRefs()` method will be publicly
available instead of requiring a JGit internal API.

Document that the packRefs() call is required in order to confirm that
the previous bug in JGitAPIImpl is still fixed.  I reverted the fix and
confirmed that without the call to `packRefs()` the bug is not shown by
the test.

Assert that the set of tags has the correct size.

src/test/java/org/jenkinsci/plugins/gitclient/JGitLightweightTagTest.java

@@ -7,14 +7,14 @@
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.hasItem;
 import static org.hamcrest.Matchers.hasProperty;
+import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.io.FileMatchers.anExistingDirectory;
 
 import hudson.EnvVars;
 import hudson.model.TaskListener;
 import hudson.plugins.git.GitObject;
 import java.io.File;
-import java.io.IOException;
 import java.io.PrintWriter;
 import java.nio.charset.StandardCharsets;
 import java.util.List;
@@ -81,10 +81,9 @@ private void createFile(final String path, final String content) throws Exceptio
         }
     }
 
-    private void packRefs() throws IOException {
+    private void packRefs() throws Exception {
         try (FileRepository repo = new FileRepository(repoRootGitDir)) {
-            org.eclipse.jgit.internal.storage.file.GC gc;
-            gc = new org.eclipse.jgit.internal.storage.file.GC(repo);
+            org.eclipse.jgit.internal.storage.file.GC gc = new org.eclipse.jgit.internal.storage.file.GC(repo);
             gc.packRefs();
         }
     }
@@ -93,7 +92,6 @@ private void packRefs() throws IOException {
     // But sometimes we want a lightweight a.k.a. non-annotated tag.
     private void lightweightTag(String tagName) throws Exception {
         try (FileRepository repo = new FileRepository(repoRootGitDir)) {
-            // Collides with implicit org.jenkinsci.plugins.gitclient.Git.
             org.eclipse.jgit.api.Git jgitAPI = org.eclipse.jgit.api.Git.wrap(repo);
             jgitAPI.tag().setName(tagName).setAnnotated(false).call();
         }
@@ -112,6 +110,7 @@ public void testGetTags_packedRefs() throws Exception {
         String annotatedTagName = "annotated_tag";
         gitClient.tag(annotatedTagName, "Tag annotation");
 
+        // Must pack the tags and other refs in order to show the JGitAPIImpl bug
         packRefs();
 
         Set<GitObject> tags = gitClient.getTags();
@@ -124,5 +123,6 @@ public void testGetTags_packedRefs() throws Exception {
                 tags,
                 hasItem(allOf(
                         hasProperty("name", equalTo(annotatedTagName)), hasProperty("SHA1", equalTo(secondCommit)))));
+        assertThat(tags, hasSize(2));
     }
 }