WIP: Improve merge request titles with external issue references

What does this MR do?

When we create a merge request with a single commit, the merge request suggested title does not include the reference to external issues (eg, JIRA ID references).

The issue has changed since it was originally reported, this is the current behavior of the system:

Use Case	Branch Name	Current Merge Request Title
1. No commits	branch-name	WIP: Branch name
2. No commits	123-branch-name	WIP: Resolve "GitLab issue title"
3. No commits	JIRA-123-branch-name	WIP: Resolve JIRA-123 "Branch name"
4. One commit	branch-name	Commit title
5. One commit	123-branch-name	Commit title
6. One commit	JIRA-123-branch-name	Commit title
7. Multiple commits	branch-name	Branch name
8. Multiple commits	123-branch-name	Resolve "GitLab issue title"
9. Multiple commits	JIRA-123-branch-name	Resolve JIRA-123 "Branch name"

This merge request fixes use case 6.

Related to #17171 (closed)