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-gpg/src/test/java/com/google/gerrit/gpg/PublicKeyStoreTest.java

@@ -149,7 +149,7 @@ public class PublicKeyStoreTest {
         RevWalk rw = new RevWalk(reader)) {
       NoteMap notes = NoteMap.read(
           reader, tr.getRevWalk().parseCommit(
-            tr.getRepository().getRef(REFS_GPG_KEYS).getObjectId()));
+            tr.getRepository().exactRef(REFS_GPG_KEYS).getObjectId()));
       String contents = new String(
           reader.open(notes.get(keyObjectId(key1.getKeyId()))).getBytes(),
           UTF_8);