RepositoryCache: don't require HEAD in git repositories

Reftable-enabled repositories don't have a file called HEAD. Check for
reftable/ instead.

This fixes repository creation on reftable in Gerrit.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I778c2be01d96aaf135affae4b457b5fe5b483bee

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/FileReftableTest.java

@@ -73,6 +73,7 @@ import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.lib.RefUpdate.Result;
 import org.eclipse.jgit.lib.ReflogEntry;
 import org.eclipse.jgit.lib.ReflogReader;
+import org.eclipse.jgit.lib.RepositoryCache;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.test.resources.SampleDataRepositoryTestCase;
 import org.eclipse.jgit.transport.ReceiveCommand;
@@ -507,6 +508,11 @@ public class FileReftableTest extends SampleDataRepositoryTestCase {
 		}
 	}
 
+	@Test
+	public void isGitRepository() {
+		assertTrue(RepositoryCache.FileKey.isGitRepository(db.getDirectory(), db.getFS()));
+	}
+
 	@Test
 	public void testRenameDestExists() throws IOException {
 		ObjectId rb = db.resolve("refs/heads/b");

org.eclipse.jgit/src/org/eclipse/jgit/lib/RepositoryCache.java

@@ -476,7 +476,8 @@ public class RepositoryCache {
 		public static boolean isGitRepository(File dir, FS fs) {
 			return fs.resolve(dir, Constants.OBJECTS).exists()
 					&& fs.resolve(dir, "refs").exists() //$NON-NLS-1$
-					&& isValidHead(new File(dir, Constants.HEAD));
+					&& (fs.resolve(dir, Constants.REFTABLE).exists()
+							|| isValidHead(new File(dir, Constants.HEAD)));
 		}
 
 		private static boolean isValidHead(File head) {