Improve readability of timestamps shown by debug-show-dir-cache

The old format is hard to read.

Change-Id: I27f3a7dbd92b26256993d27d5223b743fef70902
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -58,7 +58,7 @@ class ShowDirCache extends TextBuiltin {
 	@Override
 	protected void run() throws Exception {
 		final SimpleDateFormat fmt;
-		fmt = new SimpleDateFormat("yyyyMMdd,HHmmss.SSS");
+		fmt = new SimpleDateFormat("yyyy-MM-dd,HH:mm:ss.SSS");
 
 		final DirCache cache = db.readDirCache();
 		for (int i = 0; i < cache.getEntryCount(); i++) {