Add repository submodules api (#956)

* Add repository submodules api

Fixes #921

* Fix unit test

src/main/java/org/gitlab4j/api/RepositorySubmodulesApi.java

+package org.gitlab4j.api;
+
+import javax.ws.rs.core.Response;
+
+import org.gitlab4j.api.models.Commit;
+
+/**
+ * <p>This class provides an entry point to all the GitLab API repository submodules calls.
+ * For more information on the repository APIs see:</p>
+ *
+ * @see <a href="https://docs.gitlab.com/ee/api/repository_submodules.html">Repository Submodules API</a>
+ */
+public class RepositorySubmodulesApi extends AbstractApi {
+
+    public RepositorySubmodulesApi(GitLabApi gitLabApi) {
+        super(gitLabApi);
+    }
+
+    /**
+     * Update existing submodule reference in repository.
+     *
+     * <pre><code>GitLab Endpoint: PUT /projects/:id/repository/submodules/:submodule</code></pre>
+     *
+     * @param projectIdOrPath the project in the form of an Long(ID), String(path), or Project instance
+     * @param submodule full path to the submodule
+     * @param branch name of the branch to commit into
+     * @param commitSha full commit SHA to update the submodule to
+     * @param commitMessage commit message (optional). If no message is provided, a default is set
+     * @return the created commit
+     * @throws GitLabApiException if any exception occurs
+     */
+    public Commit updateExistingSubmoduleReference(Object projectIdOrPath, String submodule, String branch, String commitSha, String commitMessage) throws GitLabApiException {
+        GitLabApiForm formData = new GitLabApiForm()
+                .withParam("branch", branch, true)
+                .withParam("commit_sha", commitSha, true)
+                .withParam("commit_message", commitMessage);
+        Response response = put(Response.Status.OK, formData.asMap(), "projects",
+                getProjectIdOrPath(projectIdOrPath), "repository", "submodules", urlEncode(submodule));
+        return (response.readEntity(Commit.class));
+    }
+
+}

src/test/java/org/gitlab4j/api/TestRepositorySubmodulesApi.java

+package org.gitlab4j.api;
+
+import static org.gitlab4j.api.JsonUtils.compareJson;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+import static org.mockito.MockitoAnnotations.openMocks;
+
+import java.io.IOException;
+
+import javax.ws.rs.core.MultivaluedMap;
+
+import org.gitlab4j.api.models.Commit;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Captor;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+
+public class TestRepositorySubmodulesApi implements Constants {
+
+    @Mock private GitLabApi gitLabApi;
+    @Mock private GitLabApiClient gitLabApiClient;
+    @Captor private ArgumentCaptor<MultivaluedMap<String, String>> attributeCaptor;
+    private MockResponse response;
+
+    @BeforeEach
+    public void setUp() throws Exception {
+        openMocks(this);
+    }
+
+    @Test
+    public void testUpdateExistingSubmoduleReference() throws Exception {
+        init();
+        Commit result = new RepositorySubmodulesApi(gitLabApi).updateExistingSubmoduleReference(6L, "my-sub", "patch-1", "33e2ee8579fda5bc36accc9c6fbd0b4fefda9e30", "message");
+        assertNotNull(result);
+        assertTrue(compareJson(result, "commit.json"));
+    }
+
+    private void init() throws Exception, IOException {
+        response = new MockResponse(Commit.class, "commit.json", null);
+        when(gitLabApi.getApiClient()).thenReturn(gitLabApiClient);
+        when(gitLabApiClient.validateSecretToken(any())).thenReturn(true);
+        when(gitLabApiClient.put(attributeCaptor.capture(), Mockito.<Object>any())).thenReturn(response);
+    }
+}