This avoids loading authorship information for git commits, since it
relies on a GraphQL API that wasn't available before GHE v3.0. The
authorship information wasn't necessary for the merge operation anyway;
just loading the last commit OID was.
The `--json` flag accepts a list of GraphQL fields to query for and
output in JSON format. To get the list of available flags, run the
command with a blank value for `--json`. Additional `--jq` and
`--template` flags are available just like in `gh api`.
