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:

Multiple commits:

Confidential issue:

cc @DouweM @zj
Closes#14564
See merge request !3966