diff: Default arguments to HEAD, working directory

Similar to C Git, default our difference when no trees are given
to us to something that makes a tiny bit of sense to the human.

We also now support the --cached flag, and have its meaning work the
same way as C Git.

Change-Id: I2f19dad4e018404e280ea3e95ebd448a4b667f59
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/CLIText.properties

@@ -140,6 +140,7 @@ usage_alterTheDetailShown=alter the detail shown
 usage_approveDestructionOfRepository=approve destruction of repository
 usage_beMoreVerbose=be more verbose
 usage_beVerbose=be verbose
+usage_cached=compare against index
 usage_cloneRepositoryIntoNewDir=Clone a repository into a new directory
 usage_configFile=configuration file
 usage_configureTheServiceInDaemonServicename=configure the service in daemon.servicename

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

@@ -45,8 +45,12 @@
 
 package org.eclipse.jgit.pgm;
 
+import static org.eclipse.jgit.lib.Constants.HEAD;
+import static org.eclipse.jgit.lib.Constants.OBJECT_ID_STRING_LENGTH;
+
 import java.io.BufferedOutputStream;
 import java.io.PrintWriter;
+import java.text.MessageFormat;
 import java.util.List;
 
 import org.eclipse.jgit.diff.DiffEntry;
@@ -56,10 +60,14 @@ import org.eclipse.jgit.diff.RawTextIgnoreLeadingWhitespace;
 import org.eclipse.jgit.diff.RawTextIgnoreTrailingWhitespace;
 import org.eclipse.jgit.diff.RawTextIgnoreWhitespaceChange;
 import org.eclipse.jgit.diff.RenameDetector;
-import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.dircache.DirCacheIterator;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.TextProgressMonitor;
 import org.eclipse.jgit.pgm.opt.PathTreeFilterHandler;
 import org.eclipse.jgit.treewalk.AbstractTreeIterator;
+import org.eclipse.jgit.treewalk.CanonicalTreeParser;
+import org.eclipse.jgit.treewalk.FileTreeIterator;
 import org.eclipse.jgit.treewalk.filter.TreeFilter;
 import org.kohsuke.args4j.Argument;
 import org.kohsuke.args4j.Option;
@@ -69,12 +77,15 @@ class Diff extends TextBuiltin {
 	private final DiffFormatter diffFmt = new DiffFormatter( //
 			new BufferedOutputStream(System.out));
 
-	@Argument(index = 0, metaVar = "metaVar_treeish", required = true)
+	@Argument(index = 0, metaVar = "metaVar_treeish")
 	private AbstractTreeIterator oldTree;
 
-	@Argument(index = 1, metaVar = "metaVar_treeish", required = true)
+	@Argument(index = 1, metaVar = "metaVar_treeish")
 	private AbstractTreeIterator newTree;
 
+	@Option(name = "--cached", usage = "usage_cached")
+	private boolean cached;
+
 	@Option(name = "--", metaVar = "metaVar_paths", multiValued = true, handler = PathTreeFilterHandler.class)
 	private TreeFilter pathFilter = TreeFilter.ALL;
 
@@ -128,7 +139,7 @@ class Diff extends TextBuiltin {
 
 	@Option(name = "--full-index")
 	void abbrev(@SuppressWarnings("unused") boolean on) {
-		diffFmt.setAbbreviationLength(Constants.OBJECT_ID_STRING_LENGTH);
+		diffFmt.setAbbreviationLength(OBJECT_ID_STRING_LENGTH);
 	}
 
 	@Option(name = "--src-prefix", usage = "usage_srcPrefix")
@@ -153,6 +164,27 @@ class Diff extends TextBuiltin {
 	protected void run() throws Exception {
 		diffFmt.setRepository(db);
 		try {
+			if (cached) {
+				if (oldTree == null) {
+					ObjectId head = db.resolve(HEAD + "^{tree}");
+					if (head == null)
+						die(MessageFormat.format(CLIText.get().notATree, HEAD));
+					CanonicalTreeParser p = new CanonicalTreeParser();
+					ObjectReader reader = db.newObjectReader();
+					try {
+						p.reset(reader, head);
+					} finally {
+						reader.release();
+					}
+					oldTree = p;
+				}
+				newTree = new DirCacheIterator(db.readDirCache());
+			} else if (oldTree == null) {
+				oldTree = new DirCacheIterator(db.readDirCache());
+				newTree = new FileTreeIterator(db);
+			} else if (newTree == null)
+				newTree = new FileTreeIterator(db);
+
 			diffFmt.setProgressMonitor(new TextProgressMonitor());
 			diffFmt.setPathFilter(pathFilter);
 			if (detectRenames != null)