bin/git-land: fix duplicate commit body
bin/git-land: fix duplicate commit body

Commit 1a0436f77e029b14582f5b46c8fde6deebed8f35 didn't
sufficiently test the default behavior of the
commit_message field. We need to provide an explicit
empty string to avoid getting GitHub's default value.

Closes chain/chainprv#1120.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jeffomatic




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain