Use AfterCommitId to get commit for Viewed functionality (#25529) (#25612)

Backport #25529 by @sebastian-sauer

the PullHeadCommitID is not always available when the PR is merged.

Not sure if this is the best solution but in my simple tests it looks
like this fixes the problem - happy to get any feedback.

hopefully fixes https://github.com/go-gitea/gitea/issues/24813

Co-authored-by: sebastian-sauer <sauer.sebastian@gmail.com>

templates/repo/diff/box.tmpl

@@ -147,7 +147,7 @@
 										{{end}}
 									{{end}}
 									{{if $isReviewFile}}
-										<label data-link="{{$.Issue.Link}}/viewed-files" data-headcommit="{{$.PullHeadCommitID}}" class="viewed-file-form unselectable{{if $file.IsViewed}} viewed-file-checked-form{{end}}">
+										<label data-link="{{$.Issue.Link}}/viewed-files" data-headcommit="{{$.AfterCommitID}}" class="viewed-file-form unselectable{{if $file.IsViewed}} viewed-file-checked-form{{end}}">
 											<input type="checkbox" name="{{$file.GetDiffFileName}}" autocomplete="off"{{if $file.IsViewed}} checked{{end}}> {{$.locale.Tr "repo.pulls.has_viewed_file"}}
 										</label>
 									{{end}}