Fetch submodule repo before resolving commits

By default, this is turned off unless cmd.setFetch(true) is given.  It
will default to true in a future release to mimic c-git behaviour.

This is needed to prevent Eclipse from crashing with "Missing unknown
[REF]" when cloning a repo with submodules.

Bug: 470318
Change-Id: I8ae37c7c5bd2408cead8d57dd13e93e01e0e9dc1
Signed-off-by: Michael FIG <michael@fig.org>
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleUpdateCommand.java

@@ -91,6 +91,8 @@ public class SubmoduleUpdateCommand extends
 
 	private CloneCommand.Callback callback;
 
+	private boolean fetch = false;
+
 	/**
 	 * @param repo
 	 */
@@ -113,6 +115,19 @@ public class SubmoduleUpdateCommand extends
 		return this;
 	}
 
+	/**
+	 * Whether to fetch the submodules before we update them. By default, this
+	 * is set to <code>false</code>
+	 *
+	 * @param fetch
+	 * @return this command
+	 * @since 4.9
+	 */
+	public SubmoduleUpdateCommand setFetch(final boolean fetch) {
+		this.fetch = fetch;
+		return this;
+	}
+
 	/**
 	 * Add repository-relative submodule path to initialize
 	 *
@@ -161,7 +176,7 @@ public class SubmoduleUpdateCommand extends
 					continue;
 
 				Repository submoduleRepo = generator.getRepository();
-				// Clone repository is not present
+				// Clone repository if not present
 				if (submoduleRepo == null) {
 					if (callback != null) {
 						callback.cloningSubmodule(generator.getPath());
@@ -175,6 +190,18 @@ public class SubmoduleUpdateCommand extends
 					if (monitor != null)
 						clone.setProgressMonitor(monitor);
 					submoduleRepo = clone.call().getRepository();
+				} else if (this.fetch) {
+					if (callback != null) {
+						// FIXME: Do we need a new callback to tell them we're
+						// fetching?
+						callback.cloningSubmodule(generator.getPath());
+					}
+					FetchCommand fetchCommand = Git.wrap(submoduleRepo).fetch();
+					if (monitor != null) {
+						fetchCommand.setProgressMonitor(monitor);
+					}
+					configure(fetchCommand);
+					fetchCommand.call();
 				}
 
 				try (RevWalk walk = new RevWalk(submoduleRepo)) {