Description of the problem, including code/CLI snippet

After upgrading to python-gitlab 2.7.1 from 2.1.2 merge requests no longer consistently merge.

Having a large (more than 1000 character) merge commit message returns a 414 request too large, due to putting the commit message in query string.

mergeRequest.merge(merge_commit_message=closureNote, should_remove_source_branch=True)

The behaviour change occurred in python-gitlab 2.4.0:
878098b
#1121

Expected Behavior

Merge request should merge to the target branch with the (large) merge commit message and remove the existing source branch

Actual Behavior

gitlab.exceptions.GitlabMRClosedError exception occurs with 414 request-URI too large error

Specifications

• python-gitlab version: 2.7.1
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): internal gitlab version 13.11.2-ee