Surface when a branch is out-of-date with default branch

Context

https://gitlab.com/gitlab-org/foundations/ux-paper-cuts/team-tasks/-/issues/43#note_2582553037

It's been a while since it happened, but I think the scenario looks like this:

User 1 on day 1 clones a repo into myFeature1Branch containing
myClass.java
myOtherClass.java

and makes edits in myClass

User 2 on day 2 clones the same repo into myFeature2Branch and makes edits to myOtherClass, commits the changes and pushes the code to the repo.

User 1 on day 3 makes an additional change to myClass.java and then commits and pushes the code to the repo.

When we are performing a merge, of myFeature1Branch and myFeature2Branch to the main branch, we may merge feature2 first. When we merge feature1, we don't get an indicator to know that myOtherClass had been modified and the one committed with feature2 is the one that stays.

I won't have time until next week to test it if you need more detailed steps. I can try to recreate it.

Design idea

https://gitlab.com/gitlab-org/foundations/ux-paper-cuts/team-tasks/-/issues/43#note_2582905653

@mle

I think in this scenario what might be useful is to show when a branch is ahead/behind and allow the user to update or rebase the branch similarly like how we do with forks #550448 (closed)