Gitlab Mr Metadata Improvements

[ABrain7710]

Description

• The issue GitLab Data Collection Error for PR Metadata #3185 shows a NoneType exception happenening when trying to retrieve the head_sha, so I fixed this by providing a default value of None if the diff_refs are missing or head_sha or base_sha.
• Additionally the issue pointed out that the failure handler was assuming that the first paramater to the task would be repo_git, but it this task it can't be the first paramater the celery chain is passing the mr_ids to the first param in the task. To fix this I implemented a method to search the methods parameters for the repo_git parameter.

This PR fixes #
#3185

Signed commits

• Yes, I signed my commits.

[sgoggins]

LGTM

[Ulincsys]

LGTM