Add support for merged branches deletion

docs/gl_objects/branches.rst

@@ -49,3 +49,7 @@ Protect/unprotect a repository branch::
    .. code-block:: python
 
       branch.protect(developers_can_push=True, developers_can_merge=True)
+
+Delete the merged branches for a project::
+
+    project.delete_merged_branches()

gitlab/v4/objects.py

@@ -2811,6 +2811,21 @@ def delete_fork_relation(self, **kwargs):
         path = '/projects/%s/fork' % self.get_id()
         self.manager.gitlab.http_delete(path, **kwargs)
 
+    @cli.register_custom_action('Project')
+    @exc.on_http_error(exc.GitlabDeleteError)
+    def delete_merged_branches(self, **kwargs):
+        """Delete merged branches.
+
+        Args:
+            **kwargs: Extra options to send to the server (e.g. sudo)
+
+        Raises:
+            GitlabAuthenticationError: If authentication is not correct
+            GitlabDeleteError: If the server failed to perform the request
+        """
+        path = '/projects/%s/repository/merged_branches' % self.get_id()
+        self.manager.gitlab.http_delete(path, **kwargs)
+
     @cli.register_custom_action('Project')
     @exc.on_http_error(exc.GitlabCreateError)
     def star(self, **kwargs):