RefDatabase: Remove fallback exactRef implementation

This is simpler to implement than getRef.  Make it abstract so
implementers remember to override it.

Change-Id: I5f319be1fb1206d7a0142ea939dc4e1039f850ab
Signed-off-by: Jonathan Nieder <jrn@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/RefDatabaseConflictingNamesTest.java

@@ -94,6 +94,11 @@ public class RefDatabaseConflictingNamesTest {
 			return false;
 		}
 
+		@Override
+		public Ref exactRef(String name) throws IOException {
+			return null;
+		}
+
 		@Override
 		public Ref getRef(String name) throws IOException {
 			return null;

org.eclipse.jgit/.settings/.api_filters

@@ -38,6 +38,14 @@
             </message_arguments>
         </filter>
     </resource>
+    <resource path="src/org/eclipse/jgit/lib/RefDatabase.java" type="org.eclipse.jgit.lib.RefDatabase">
+        <filter comment="class is extended by extenders but not clients of the API" id="421650549">
+            <message_arguments>
+                <message_argument value="org.eclipse.jgit.lib.RefDatabase"/>
+                <message_argument value="exactRef(String)"/>
+            </message_arguments>
+        </filter>
+    </resource>
     <resource path="src/org/eclipse/jgit/revwalk/DepthWalk.java" type="org.eclipse.jgit.revwalk.DepthWalk">
         <filter id="403804204">
             <message_arguments>

org.eclipse.jgit/src/org/eclipse/jgit/lib/RefDatabase.java

@@ -290,13 +290,7 @@ public abstract class RefDatabase {
 	 * @since 4.1
 	 */
 	@Nullable
-	public Ref exactRef(String name) throws IOException {
-		Ref ref = getRef(name);
-		if (ref == null || !name.equals(ref.getName())) {
-			return null;
-		}
-		return ref;
-	}
+	public abstract Ref exactRef(String name) throws IOException;
 
 	/**
 	 * Read the specified references.