Stacked merge requests in the UI

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Previously, when you split a large change into smaller merge requests that build on each other, the UI gave no signal that they were related. Authors and reviewers had to track the sequence manually.

GitLab now detects stacked merge requests automatically and shows them in the merge request header. A merge request joins a stack when it targets another open merge request's source branch, or when another open merge request targets its source branch. The stack control next to the source branch shows the current position (for example, 1 of 2) and lets you jump to any other merge request in the stack.

To create stacked merge requests from the command line, use stacked diffs in the GitLab CLI.