I ended up creating a temporary branch from the target branch (B). Then, I merged my original merge commit into the temporary branch, which re-applied the previous merge but only required me to resolve the new conflicts caused by updates to branch B. After resolving those conflicts and committing the changes, I switched back to the B branch and merged the updates from the temporary branch into it. Finally, I pushed the updated B branch to Gerrit, and the changes were successfully accepted.