Format submodule links during differences

Instead of crashing, output a submodule link with the simple
"Subproject commit $fullid\n" syntax used by C Git.

Change-Id: Iae8646941683fb19b73fb038217d2e3bf5f77fa9
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java

@@ -46,6 +46,7 @@ package org.eclipse.jgit.diff;
 
 import static org.eclipse.jgit.lib.Constants.encode;
 import static org.eclipse.jgit.lib.Constants.encodeASCII;
+import static org.eclipse.jgit.lib.FileMode.GITLINK;
 
 import java.io.IOException;
 import java.io.OutputStream;
@@ -232,6 +233,16 @@ public class DiffFormatter {
 		out.write(encode("--- " + oldName + '\n'));
 		out.write(encode("+++ " + newName + '\n'));
 
+		if (ent.getOldMode() == GITLINK || ent.getNewMode() == GITLINK) {
+			if (ent.getOldMode() == GITLINK) {
+				out.write(encodeASCII("-Subproject commit "
+						+ ent.getOldId().name() + "\n"));
+			}
+			if (ent.getNewMode() == GITLINK) {
+				out.write(encodeASCII("+Subproject commit "
+						+ ent.getNewId().name() + "\n"));
+			}
+		} else {
 			byte[] aRaw = open(ent.getOldMode(), ent.getOldId());
 			byte[] bRaw = open(ent.getNewMode(), ent.getNewId());
 
@@ -244,6 +255,7 @@ public class DiffFormatter {
 				formatEdits(a, b, new MyersDiff(a, b).getEdits());
 			}
 		}
+	}
 
 	/**
 	 * Construct a RawText sequence for use with {@link MyersDiff}.