The branches API endpoint cannot handle ref names with slashes (e.g.
pull-request/2223). When the API returns a 404, gh api writes the error
JSON to stdout before exiting non-zero, so the || fallback never fires.
This causes mirror_sha to contain raw JSON like {"message":"Branch not
found",...} and the comment displays `{"messa` as the SHA.
Switch to the git/ref/heads/ endpoint which handles slashes natively,
and add a regex guard to reset mirror_sha to empty when it does not
look like a valid 40-char hex SHA.
Signed-off-by: Roland Huß <rhuss@redhat.com>
Roland Huss
3ff15a1657