Resolve "Squash commit message does not honor regex settings"

What does this MR do and why?

Apply push rules to squash commit messages

Contributes to: #345359 (closed)

Squash commit messages were not being considered

How to set up and validate locally

1. Create a project with merge method set to fast-forward merge
2. Add a push rule for required expression in commit message e.g TICKET-
3. Create a merge request
4. Edit squash commit message such that it does not contain the required expression
5. See the a reasonable error message in the MR widget
6. Edit squash commit message to something containing the required expression and merge
7. See the merge request is successful

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #345359 (closed)