Added get commits by path method (#73)

* Added get commits by path method

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

@@ -87,6 +87,26 @@ public class CommitsApi extends AbstractApi {
         return (response.readEntity(new GenericType<List<Commit>>() {}));
     }
 
+    /**
+     * Get a list of file commits in a project
+     *
+     * GET /projects/:id/repository/commits?path=:file_path
+     *
+     * @param projectId the project ID to get the list of commits for
+     * @param ref the name of a repository branch or tag or if not given the default branch
+     * @param path the path to file of a project
+     * @return a list containing the commits for the specified project ID and file
+     * @throws GitLabApiException GitLabApiException if any exception occurs during execution
+     */
+    public List<Commit> getCommits(int projectId, String ref, String path) throws GitLabApiException {
+        Form formData = new GitLabApiForm()
+                .withParam("ref_name", ref)
+                .withParam(PER_PAGE_PARAM, getDefaultPerPage());
+        String pathArg = (path == null || path.isEmpty()) ? "commits" : "commits" + "?path=" + path;
+        Response response = get(Response.Status.OK, formData.asMap(), "projects", projectId, "repository", pathArg);
+        return (response.readEntity(new GenericType<List<Commit>>() {}));
+    }
+
     /**
      * Get a list of repository commits in a project.
      *

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

@@ -147,4 +147,15 @@ public class TestCommitsApi {
         assertNotNull(pager);
         assertTrue(pager.getTotalItems() > 0);
     }
-}
+
+    @Test
+    public void testCommitsByPath() throws GitLabApiException {
+        Project project = gitLabApi.getProjectApi().getProject(TEST_NAMESPACE, TEST_PROJECT_NAME);
+
+        CommitsApi commitsApi = gitLabApi.getCommitsApi();
+        List<Commit> commits = commitsApi.getCommits(project.getId(), "master",
+            "README");
+        assertNotNull(commits);
+        assertTrue(commits.size() > 0);
+    }
+}
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