Ignore submodule commits during checkout

Submodules are supposed to be handled by separate operations, so
we should ignore them on checkout, just like C Git does.

This fix does not add submodule support. We just try harder
to ignore them.

Bug: 343566
Change-Id: I2c5ae1024ea7bb57adf27072da6acc9643018eda
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -417,7 +417,13 @@ public class DirCacheCheckout {
 			if (!file.getParentFile().mkdirs()) {
 				// ignore
 			}
+
 			DirCacheEntry entry = dc.getEntry(path);
+
+			// submodules are handled with separate operations
+			if (FileMode.GITLINK.equals(entry.getRawMode()))
+				continue;
+
 			checkoutEntry(repo, file, entry);
 		}