chore: add and document optional parameters for get MR

Add and document (some of the) optional parameters that can be done
for a `project.merge_requests.get()`

Closes #1775

docs/gl_objects/merge_requests.rst

@@ -186,6 +186,14 @@ Attempt to rebase an MR::
 
     mr.rebase()
 
+Get status of a rebase for an MR::
+
+    mr = project.mergerequests.get(mr_id, include_rebase_in_progress=True)
+    print(mr.rebase_in_progress, mr.merge_error)
+
+For more info see:
+https://docs.gitlab.com/ee/api/merge_requests.html#rebase-a-merge-request
+
 Attempt to merge changes between source and target branch::
 
     response = mr.merge_ref()

gitlab/v4/objects/merge_requests.py

@@ -396,6 +396,11 @@ class ProjectMergeRequestManager(CRUDMixin, RESTManager):
     _path = "/projects/{project_id}/merge_requests"
     _obj_cls = ProjectMergeRequest
     _from_parent_attrs = {"project_id": "id"}
+    _optional_get_attrs = (
+        "render_html",
+        "include_diverged_commits_count",
+        "include_rebase_in_progress",
+    )
     _create_attrs = RequiredOptional(
         required=("source_branch", "target_branch", "title"),
         optional=(