Auto-set title for branches created from issues

This sets the title for a new MR to 'Resolves "$issue-title"' when:
- The source branch for the MR begins with a value iid.
- The MR has more than one commit in its diff (if there's one commit, keep using the commit's first line).
- The iid does not point to a confidential issue.

Single commit:

![A single commit uses the commit title](/uploads/cd34f59cd67f095c3034fae07950f8b5/image.png)

Multiple commits:

![Multiple commits use the issue title](/uploads/a322c406ddd56913c5aebd88d16e5a5e/image.png)

Confidential issue:

![A confidential issue uses the branch name](/uploads/7ae9b79de5f6101ced46802f3c3a6e71/image.png)

cc @DouweM @zj 

Closes #14564

See merge request !3966