Add family for the quick diff initialize job

The quick diff initialization job currently has no family,
making it difficult to wait on it during tests.
During JDT UI SaveParticipantTest we observe the job running
in parallel to the test, potentially running into errors
due to file operations done in the test.

This change adds a family that tests can use to wait on the job.

Fixes: #4172

See also: eclipse-jdt/eclipse.jdt.ui#79

Author: trancexpress

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/texteditor/quickdiff/DocumentLineDiffer.java

@@ -134,6 +134,11 @@ public String[] getOriginalText() {
 		}
 	}
 
+	/**
+	 * Family for the quick diff initialize job.
+	 */
+	public static final Object QUICKDIFF_INITIALIZE_FAMILY = new Object();
+
 	/** Tells whether this class is in debug mode. */
 	private static boolean DEBUG= "true".equalsIgnoreCase(Platform.getDebugOption("org.eclipse.ui.workbench.texteditor/debug/DocumentLineDiffer"));  //$NON-NLS-1$//$NON-NLS-2$
 
@@ -550,6 +555,11 @@ protected synchronized void initialize() {
 				: QuickDiffMessages.quickdiff_initialize;
 		fInitializationJob= new Job(jobName) {
 
+			@Override
+			public boolean belongsTo(Object family) {
+				return family == QUICKDIFF_INITIALIZE_FAMILY;
+			}
+
 			/*
 			 * This is run in a different thread. As the documents might be synchronized, never ever
 			 * access the documents in a synchronized section or expect deadlocks. See