[land-bot] Fix git HEAD after checking revision

Summary:
D7455 breaks the land bot because arc land --hold
detaches git HEAD when it's expected to be on branch master.

Test Plan:

CONDUIT_TOKEN=<token> ./apply-revision.sh Dxxxx
git log

Pre-patch git log will show HEAD in a detached state.
Post-patch, git log will show HEAD pointing as master with
one new commit, as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7478