[land-bot] Pass committer name/email to land bot endpoint

Summary: The land endpoint requires these fields now.

Test Plan:
Used to land D6796:
arc land-via-bot --revision D6796

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6799