Implement exactRef in FilteredRefDatabase

The default implementation of exactRef is slower than it needs to be
implement exactRef directly to simplify and get a potential speedup.

Change-Id: If9fd1dbdedeefb863349d85bf353c2bda43e304d

src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java b/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java
index 6ddd490..098910c 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/gitiles/FilteredRepository.java
@@ -191,6 +191,15 @@
     }
 
     @Override
+    public Ref exactRef(String name) throws IOException {
+      Ref ref = delegate.exactRef(name);
+      if (ref == null) {
+        return null;
+      }
+      return refFilter.filter(ImmutableMap.of(name, ref), true).get(name);
+    }
+
+    @Override
     public Ref getRef(String name) throws IOException {
       Ref ref = delegate.getRef(name);
       if (ref == null) {