Fix deprecation warnings from latest JGit update

- FS is no longer required for LockFile constructor.
 - TestRepository#getClock was renamed getDate.
 - Replace usages of RefDatabase#getRef with either exactRef, when we
   know we can bypass the full lookup path, or findRef, where the
   input is user-provided and we might need to add an implicit prefix.

Change-Id: I6309b91ccf99ffaa724be273ddae0d9857b825f9

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/accounts/AccountIT.java

@@ -102,7 +102,7 @@ public class AccountIT extends AbstractDaemonTest {
   @After
   public void clearPublicKeyStore() throws Exception {
     try (Repository repo = repoManager.openRepository(allUsers)) {
-      Ref ref = repo.getRef(REFS_GPG_KEYS);
+      Ref ref = repo.exactRef(REFS_GPG_KEYS);
       if (ref != null) {
         RefUpdate ru = repo.updateRef(REFS_GPG_KEYS);
         ru.setForceUpdate(true);

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java

@@ -106,7 +106,7 @@ public class ChangeIT extends AbstractDaemonTest {
     gApi.changes().id(changeId).get();
 
     BranchInput b = new BranchInput();
-    b.revision = repo().getRef("HEAD").getObjectId().name();
+    b.revision = repo().exactRef("HEAD").getObjectId().name();
     gApi.projects()
         .name(project.get())
         .branch("other")

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java

@@ -377,7 +377,7 @@ public class RevisionIT extends AbstractDaemonTest {
 
   @Test
   public void mergeable() throws Exception {
-    ObjectId initial = repo().getRef(HEAD).getLeaf().getObjectId();
+    ObjectId initial = repo().exactRef(HEAD).getLeaf().getObjectId();
 
     PushOneCommit push1 =
         pushFactory.create(db, admin.getIdent(), testRepo, PushOneCommit.SUBJECT,