[land-bot] Allow users to specify the branch/commit in order to determine the revision to land

Summary:
Specifying a branch does not work as expected. When working on stacked revisions, users
expect something like arc land-via-bot HEAD~ to land the revision associated with the commit
at HEAD~. This patch fixes that behavior.

Test Plan:

git checkout <some-stacked-revision>
git cherry-pick <this-patch>
arc land-via-bot --preview
arc land-via-bot --preview HEAD~
arc land-via-bot --preview HEAD~2
arc land-via-bot --hold
arc land-via-bot --hold HEAD~
arc land-via-bot --hold HEAD~2

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7959