Do not backport PRs closed ("merged") because of other PRs getting merged

[kmaehashi]

cupy/cupy#8460 (comment)

It seems that the backport tool does not cover this case: two PRs with to-be-backported label exist and one closed because of another one getting merged.

cupy/cupy#8221 automatically closed as "merged" because cupy/cupy#8412 got merged, which includes all commits from 8221.

This case can be detected by checking merged_by field from the pull-request API response. If it is null (like 8221), it indicates the PR was not merged but all commits in the PR got merged in another PR.