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

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 #24813

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}}