fetch: Accept any SHA-1 on lhs of refspec

Allow fetch to accept a SHA-1 on the left hand side of a RefSpec,
enabling callers to pass a specific SHA-1 they want that may not have
been advertised by the remote repository. This can be passed along to
the network protocol to be sent in a "want" line.

Rest of the plumbing only cares about the ObjectId of the Ref in
the askFor map, so make up a fake name using ObjectId.name() to
pass the desired ObjectId into the network code.

Change-Id: I620a189f3de005c403aa68b7d0442d6aa94e6056

org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java

@@ -74,6 +74,7 @@ import org.eclipse.jgit.lib.BatchRefUpdate;
 import org.eclipse.jgit.lib.BatchingProgressMonitor;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
 import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.RefDatabase;
@@ -360,13 +361,20 @@ class FetchProcess {
 
 	private void expandSingle(final RefSpec spec, final Set<Ref> matched)
 			throws TransportException {
-		final Ref src = conn.getRef(spec.getSource());
+		String want = spec.getSource();
+		if (ObjectId.isId(want)) {
+			want(ObjectId.fromString(want));
+			return;
+		}
+
+		Ref src = conn.getRef(want);
 		if (src == null) {
-			throw new TransportException(MessageFormat.format(JGitText.get().remoteDoesNotHaveSpec, spec.getSource()));
+			throw new TransportException(MessageFormat.format(JGitText.get().remoteDoesNotHaveSpec, want));
 		}
-		if (matched.add(src))
+		if (matched.add(src)) {
 			want(src, spec);
 		}
+	}
 
 	private Collection<Ref> expandAutoFollowTags() throws TransportException {
 		final Collection<Ref> additionalTags = new ArrayList<>();
@@ -440,6 +448,11 @@ class FetchProcess {
 		fetchHeadUpdates.add(fhr);
 	}
 
+	private void want(ObjectId id) {
+		askFor.put(id,
+				new ObjectIdRef.Unpeeled(Ref.Storage.NETWORK, id.name(), id));
+	}
+
 	private TrackingRefUpdate createUpdate(RefSpec spec, ObjectId newId)
 			throws TransportException {
 		Ref ref = localRefs().get(spec.getDestination());