Backport #39613 by @dziulatex
Fixes#39598
When the base branch has "Block merge if the pull request is outdated"
enabled, an AGit pull request that fell behind could never be unblocked:
merging the base branch into the topic and pushing it again to the same
`refs/for/<branch>/<topic>` left the PR "blocked because it's outdated"
forever.
The outdated block reads the stored `commits_behind`. Pushing a branch
refreshes it for GitHub-flow PRs (`AddTestPullRequestTask`), but the
AGit update path in `services/agit` moved `refs/pull/N/head` without
recomputing it, so the value only changed when the base branch was
pushed, which is the event that makes a PR behind again.
The AGit update path now calls `syncCommitDivergence` in
`UpdateRefForAgit`, the same as AGit PR creation (`NewPullRequest`)
already does.
Signed-off-by: dziulatex <paweldziurasoftware@yahoo.com>
Co-authored-by: dziulatex <38464243+dziulatex@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
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