Summary:
`arc land` with no args tries to land onto the `master` branch. This fixes said issue.
Also cleanup the JSON indentation.

Test Plan: `arc land` now picks `main` as the branch to land onto by default.

Reviewers: michelle, zasgar, #engineering

Reviewed By: zasgar, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D6564

GitOrigin-RevId: 4236dbcd9c2cba7def49a99c1e748e9e027a9455