[CLI] Add option --millis / -m to debug-show-dir-cache command

This is useful when comparing mtime displayed by 
    $ jgit debug-show-dir-cache -m
with mtime displayed by 
    $ git ls-files --debug 
or 
    $ stat "%m"

Change-Id: Id133ebe6f6093a56a6a6645e1c5bb18752fb2fd0
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -317,6 +317,7 @@ usage_showRefNamesMatchingCommits=Show ref names matching commits
 usage_showPatch=display patch
 usage_showRefNamesMatchingCommits=Show ref names matching commits
 usage_showNotes=Add this ref to the list of note branches from which notes are displayed
+usage_showTimeInMilliseconds=Show mtime in milliseconds
 usage_squash=Squash commits as if a real merge happened, but do not make a commit or move the HEAD.
 usage_srcPrefix=show the source prefix instead of "a/"
 usage_symbolicVersionForTheProject=Symbolic version for the project

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/debug/ShowDirCache.java

@@ -56,26 +56,35 @@ import org.eclipse.jgit.dircache.DirCacheEntry;
 import org.eclipse.jgit.lib.FileMode;
 import org.eclipse.jgit.pgm.Command;
 import org.eclipse.jgit.pgm.TextBuiltin;
+import org.kohsuke.args4j.Option;
 
 @Command(usage = "usage_ShowDirCache")
 class ShowDirCache extends TextBuiltin {
+
+	@Option(name = "--millis", aliases = { "-m" }, usage = "usage_showTimeInMilliseconds")
+	private boolean millis = false;
+
 	@Override
 	protected void run() throws Exception {
 		final SimpleDateFormat fmt;
-		fmt = new SimpleDateFormat("yyyy-MM-dd,HH:mm:ss.SSS");
+		fmt = new SimpleDateFormat("yyyy-MM-dd,HH:mm:ss.SSS"); //$NON-NLS-1$
 
 		final DirCache cache = db.readDirCache();
 		for (int i = 0; i < cache.getEntryCount(); i++) {
 			final DirCacheEntry ent = cache.getEntry(i);
 			final FileMode mode = FileMode.fromBits(ent.getRawMode());
 			final int len = ent.getLength();
-			final Date mtime = new Date(ent.getLastModified());
+			long lastModified = ent.getLastModified();
+			final Date mtime = new Date(lastModified);
 			final int stage = ent.getStage();
 
 			outw.print(mode);
 			outw.format(" %6d", valueOf(len)); //$NON-NLS-1$
 			outw.print(' ');
-			outw.print(fmt.format(mtime));
+			if (millis)
+				outw.print(lastModified);
+			else
+				outw.print(fmt.format(mtime));
 			outw.print(' ');
 			outw.print(ent.getObjectId().name());
 			outw.print(' ');