Fixes#39598
The AGit update path in `services/agit` moved `refs/pull/N/head` without
recomputing commits_behind.
The AGit update path now calls `syncCommitDivergence` in
`UpdateRefForAgit`, the same as AGit PR creation (`NewPullRequest`)
already does.
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Signed-off-by: dziulatex <paweldziurasoftware@yahoo.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>