Fix providing an explicit branch/ref to land-via-bot

Summary: The wildcard argument gives an array, not one object.

Test Plan:
Pre-patch, this gives a backtrace:
arc land-via-bot master --hold
Post-patch, this correctly identifies the branch and associated revision:

arc patch <this-patch>
arc land-via-bot HEAD --hold

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6969