fix(repo): surface unrelated histories on Sync Fork (#39258)

Sync Fork already maps merge conflicts to a JSON error. Unrelated
histories still went through `ServerError`, so the UI showed a 500 HTML
snippet instead of the same user-facing message PR merge already uses
(`repo.pulls.unrelated_histories`).

The API path returned 500 for the same git error; PR merge returns 409.
Match that.

Fixes #36772

AI assistance was used to locate the handler gap and draft the mapping.
I reviewed and take responsibility for the change.

Signed-off-by: Zhaoqi Xu <lzy00419@outlook.com>

routers/web/repo/branch.go

@@ -240,6 +240,9 @@ func MergeUpstream(ctx *context.Context) {
 		} else if pull_service.IsErrMergeConflicts(err) {
 			ctx.JSONError(ctx.Tr("repo.pulls.merge_conflict"))
 			return
+		} else if pull_service.IsErrMergeUnrelatedHistories(err) {
+			ctx.JSONError(ctx.Tr("repo.pulls.unrelated_histories"))
+			return
 		}
 		ctx.ServerError("MergeUpstream", err)
 		return