Update tags on fetch if --tags or tag refspec specified

When either --tags or a tag ref is explicitly specified on fetch, C Git
updates existing local tags if they are different.

Before this change, JGit returned REJECTED in such a case. Now it
updates it and returns FORCED.

Example:

    % mkdir a
    % cd a
    % git init -q
    % touch test.txt
    % git add test.txt
    % git commit -q -m 'Initial'
    % git tag v1
    % cd ..
    % git clone -q a b
    % cd a
    % echo Test > test.txt
    % git commit -q -a -m 'Second'
    % git tag -f v1
    Updated tag 'v1' (was bc85c08)
    % cd ../b
    % git fetch --tags
     - [tag update]      v1         -> v1

Bug: 388095
Change-Id: I5d5494c2ad1a2cdb8e9e614d3de445289734edfe

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/FetchCommandTest.java

@@ -52,8 +52,10 @@ import java.util.Collection;
 import org.eclipse.jgit.api.errors.GitAPIException;
 import org.eclipse.jgit.api.errors.JGitInternalException;
 import org.eclipse.jgit.junit.RepositoryTestCase;
+import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.StoredConfig;
 import org.eclipse.jgit.revwalk.RevCommit;
@@ -168,4 +170,27 @@ public class FetchCommandTest extends RepositoryTestCase {
 
 		assertEquals(originalId, db.resolve(tagName));
 	}
+
+	@Test
+	public void fetchWithExplicitTagsShouldUpdateLocal() throws Exception {
+		final String tagName = "foo";
+		remoteGit.commit().setMessage("commit").call();
+		Ref tagRef1 = remoteGit.tag().setName(tagName).call();
+
+		RefSpec spec = new RefSpec("refs/heads/*:refs/remotes/origin/*");
+		git.fetch().setRemote("test").setRefSpecs(spec)
+				.setTagOpt(TagOpt.AUTO_FOLLOW).call();
+		assertEquals(tagRef1.getObjectId(), db.resolve(tagName));
+
+		remoteGit.commit().setMessage("commit 2").call();
+		Ref tagRef2 = remoteGit.tag().setName(tagName).setForceUpdate(true)
+				.call();
+
+		FetchResult result = git.fetch().setRemote("test").setRefSpecs(spec)
+				.setTagOpt(TagOpt.FETCH_TAGS).call();
+		TrackingRefUpdate update = result.getTrackingRefUpdate(Constants.R_TAGS
+				+ tagName);
+		assertEquals(RefUpdate.Result.FORCED, update.getResult());
+		assertEquals(tagRef2.getObjectId(), db.resolve(tagName));
+	}
 }

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

@@ -412,7 +412,7 @@ class FetchProcess {
 
 	private void wantTag(final Ref r) throws TransportException {
 		want(r, new RefSpec().setSource(r.getName())
-				.setDestination(r.getName()));
+				.setDestination(r.getName()).setForceUpdate(true));
 	}
 
 	private void want(final Ref src, final RefSpec spec)