fix(pull): refresh commits behind when an AGit pull request is updated (#39613) (#39625)

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When the base branch has "Block merge if the pull request is outdated"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` left the PR "blocked because it's outdated"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The outdated block reads the stored `commits_behind`.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>

services/agit/agit.go

@@ -252,7 +252,7 @@ func ProcReceive(ctx context.Context, repo *repo_model.Repository, gitRepo *git.
 		oldHeadCommitID := pr.HeadCommitID
 
 		pr.HeadCommitID = opts.NewCommitIDs[i]
-		if err = pull_service.UpdateRef(ctx, pr); err != nil {
+		if err = pull_service.UpdateRefForAgit(ctx, pr); err != nil {
 			return nil, fmt.Errorf("failed to update pull ref. Error: %w", err)
 		}