transport-helper: warn when export-marks file cannot be finalized
When push_refs_with_export() finalizes a successful push, it writes the fast-export marks file to a .tmp sibling and rename()s it into place. The return value of rename() is currently ignored. If the rename fails (permission denied, full disk, or an antivirus product locking the destination on Windows), the .tmp file is left behind and the existing export_marks file remains stale; the next fast-export operation that resumes from it then silently operates on inconsistent bookkeeping. The push itself succeeded by that point, so promoting this to a fatal error would be inappropriate. Emit warning_errno() naming both paths so the user can recover manually, and keep returning 0. Flagged by Coverity as CID 1427723 ("Unchecked return value"). Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>