Use the appropriate constant for ".git"

We have two constants with the same content. DOT_GIT is intended
for the git repository below the work tree, while DOT_GIT_EXT is
the ".git" directory extension usually associated with bare
repositories.

Change-Id: I0946b4beb2d1c3af289ddbbb5641d2f4e4c49d3f
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

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

@@ -243,7 +243,7 @@ class DiffAlgorithms extends TextBuiltin {
 		if (db.getDirectory() != null) {
 			String name = db.getDirectory().getName();
 			File parent = db.getDirectory().getParentFile();
-			if (name.equals(Constants.DOT_GIT_EXT) && parent != null)
+			if (name.equals(Constants.DOT_GIT) && parent != null)
 				name = parent.getName();
 			out.println(name + ": start at " + startId.name());
 		}

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

@@ -342,7 +342,7 @@ class TextHashFunctions extends TextBuiltin {
 		if (db.getDirectory() != null) {
 			String name = db.getDirectory().getName();
 			File parent = db.getDirectory().getParentFile();
-			if (name.equals(Constants.DOT_GIT_EXT) && parent != null)
+			if (name.equals(Constants.DOT_GIT) && parent != null)
 				name = parent.getName();
 			out.println(name + ":");
 		}