Rename GitPersonSubject#creationDate to date

The subject method name should match the field name.

Plus, there is no clear "creation date" for a git identity: it sounds
like it refers to the date the PersonIdent instance was created, rather
than the date _stored in_the PersonIdent. But that is never something we
care about.

It could also be associated with with the "creation date" of a commit,
but that would also be ambiguous: it could refer either to the original
iteration of the commit (the author timestamp), or the current commit
object's creation date (the commit timestamp).

Let's just stick with "date".

Change-Id: I40d80558aeb8cb5cbad754f081e76fff616ad878

java/com/google/gerrit/extensions/common/testing/GitPersonSubject.java

@@ -33,7 +33,7 @@ public class GitPersonSubject extends Subject<GitPersonSubject, GitPerson> {
     super(failureMetadata, gitPerson);
   }
 
-  public ComparableSubject<?, Timestamp> creationDate() {
+  public ComparableSubject<?, Timestamp> date() {
     isNotNull();
     GitPerson gitPerson = actual();
     return Truth.assertThat(gitPerson.date).named("creationDate");