Revision: Add helper method for comparison to Revision.NULL

When Error Prone checks are enabled, comparing to Revision.NULL
with the == or != operator results in a warning:

  [ReferenceEquality] Comparison using reference equality instead of value equality

The NULL instance is used as a sentinel, so comparing with reference
equality is correct.

Rather than suppressing the warning at every place in the code, add
a helper method that can be used, and suppress the warning only there.

Note that the "ReferenceEquality" warning is Error Prone specific and
suppressing it results in an "Unsupported @SuppressWarnings" warning.

Change-Id: Ia50404f76f88c405b9298a645760c019384c92ff

java/com/google/gitiles/RevisionParser.java

diff --git a/java/com/google/gitiles/RevisionParser.java b/java/com/google/gitiles/RevisionParser.java
index 89311a3..d530cdd 100644
--- a/java/com/google/gitiles/RevisionParser.java
+++ b/java/com/google/gitiles/RevisionParser.java
@@ -225,7 +225,7 @@
     if (!cache.isVisible(repo, walk, access, id)) {
       return false;
     }
-    if (result.getOldRevision() != null && result.getOldRevision() != Revision.NULL) {
+    if (result.getOldRevision() != null && !Revision.isNull(result.getOldRevision())) {
       return cache.isVisible(repo, walk, access, result.getOldRevision().getId(), id);
     }
     return true;