diff: Add "--staged" as alias to "--cached"

see: https://git-scm.com/docs/git-diff

"--staged is a synonym of --cached"

Change-Id: Ie73f6b3d3b7179c339151cebab98f9ddee6aaf49
Signed-off-by: Andre Bossert <andre.bossert@siemens.com>

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java

@@ -87,7 +87,7 @@ class Diff extends TextBuiltin {
 	@Argument(index = 1, metaVar = "metaVar_treeish")
 	private AbstractTreeIterator newTree;
 
-	@Option(name = "--cached", usage = "usage_cached")
+	@Option(name = "--cached", aliases = { "--staged" }, usage = "usage_cached")
 	private boolean cached;
 
 	@Option(name = "--", metaVar = "metaVar_paths", handler = PathTreeFilterHandler.class)