Enable and fix more Eclipse warnings

Enable and fix warnings:

- Missing `hashCode` method on classes that define `equals`
- Redundant null check

Also enable "possible accidental boolean assignment" warning.  There
are currently no instances of this but it might be useful to enable.

Change-Id: I7659ae66bedb6a17e088c90d2165282b7fe33695

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/LineMapper.java

@@ -17,6 +17,7 @@ package com.google.gerrit.client.diff;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 
 /** Helper class to handle calculations involving line gaps. */
 class LineMapper {
@@ -192,6 +193,11 @@ class LineMapper {
       return false;
     }
 
+    @Override
+    public int hashCode() {
+      return Objects.hash(this);
+    }
+
     @Override
     public String toString() {
       return line + " " + aligned;